Add adding_to_third_party.md to //docs

Create adding_to_third_party.md to document the process of introducing
new 3rd party libraries to Chromium. Original source of information:
https://www.chromium.org/developers/adding-3rd-party-libraries

BUG=674221

Review-Url: https://codereview.chromium.org/2577123003
Cr-Commit-Position: refs/heads/master@{#439181}

docs/adding_to_third_party.md

+# Adding third_party Libraries
+
+[TOC]
+
+Using third party code can save time and is consistent with our values - no need
+to reinvent the wheel! We put all code that isn't written by Chromium developers
+into src/third_party (even if you end up modifying just a few functions). We do
+this to make it easy to track license compliance, security patches, and supply
+the right credit and attributions. It also makes it a lot easier for other
+projects that embed our code to track what is Chromium licensed and what is
+covered by other licenses.
+
+## Get the Code
+
+When you find code you want to use, get it. This often means downloading: from
+Sourceforge, from the hosting feature of Google Code, or from somewhere else.
+Sometimes it can mean negotiating a license with another company and receiving
+the code another way. Please describe the source in the README.chromium file,
+described below.  For security reasons, please retrieve the code as securely as
+you can, using HTTPS and GPG signatures if available.  If retrieving a tarball,
+please do not check the tarball itself into the tree, but do list the source and
+the SHA-512 hash (for verification) in the README.chromium and Change List. The
+SHA-512 hash can be computed via the `shasum (sha512sum)` or `openssl`
+utilities.  If retrieving from a git repository, please list the SHA-512 hash.
+
+## Put the Code in (the right) third_party
+
+By default, all code should be checked into
+[src/third_party](http://src.chromium.org/viewvc/chrome/trunk/src/third_party/).
+It is OK to have third_party subdirectories that are not top-level (e.g.
+src/net/third_party), but don't add more third_party directories than necessary.
+
+## Document the Code's Context
+
+### Add OWNERS
+
+Your OWNERS file must include 2 Chromium developer accounts. This will ensure
+accountability for maintenance of the code over time. While there isn't always
+an ideal or obvious set of people that should go in OWNERS, this is critical for
+first-line triage of any issues that crop up in the code.
+
+As an OWNER, you're expected to:
+
+* Remove the dependency when/if it is no longer needed
+* Update the dependency when a security or stability bug is fixed upstream
+* Help ensure the Chrome feature that uses the dependency continues to use the
+  dependency in the best way, as the feature and the dependency change over
+  time.
+
+### Add a README.chromium
+
+You need a README.chromium file with information about the project from which
+you're re-using code. See
+[README.chromium.template](http://src.chromium.org/viewvc/chrome/trunk/src/third_party/README.chromium.template)
+for a list of fields to include. A presubmit check will check this has the right
+format.
+
+### Add a LICENSE file and run related checks
+
+You need a LICENSE file. Example:
+[third_party/libjpeg/LICENSE](http://src.chromium.org/viewvc/chrome/trunk/src/third_party/libjpeg/LICENSE?revision=42288&view=markup).
+
+Run the following scripts:
+
+* `src/tools/licenses.py scan` - This will complain about incomplete or missing
+  data for third_party checkins. We use 'licenses.py credits' to generate the
+  about:credits page in Google Chrome builds.
+* `src/tools/checklicenses/checklicenses.py` - See below for info how to handle
+  possible failures.
+* If you are adding code that will be present in the content layer, please make
+  sure that the license used is compliant with Android tree requirements because
+  this code will also be used in Android WebView. You need to run
+  `src/android_webview/tools/webview_licenses.py scan`
+
+See the ["Odds n' Ends"](adding_to_third_party.md#Odds-n_Ends) Section below if
+you run into any failures running these.
+
+If the library will never be shipped as a part of Chrome (e.g. build-time tools,
+testing tools), make sure to set "License File" as "NOT_SHIPPED" so that the
+license is not included in about:credits page.
+
+### Modify DEPS
+
+If the code is applicable and will be compiled on all supported Chromium
+platforms (Windows, Mac, Linux, ChromeOS, iOS, Android), check it in to
+[src/third_party](http://src.chromium.org/viewvc/chrome/trunk/src/third_party/). 
+
+If the code is only applicable to certain platforms, check it in to
+[src/third_party](http://src.chromium.org/viewvc/chrome/trunk/src/third_party/)
+and add an entry to the
+[DEPS](http://src.chromium.org/viewvc/chrome/trunk/src/DEPS) file that causes
+the code to be checked out from src/third_party into src/third_party by gclient.
+
+_Explanation: Checking it into src/third_party causes all developers to need to
+check out your code. This wastes disk space cause syncing to take longer for
+developers that don't need your code. When all platforms really do need the
+code, checking it in to src/third_party allows some slight improvements over
+DEPS._
+
+As for specifying the path where the library is fetched, a path like
+`src/third_party/<project_name>/src` is highly recommended so that you can put
+the file like OWNERS or README.chromium at `third_party/<project_name>`. If you
+have a wrong path in DEPS and want to change the path of the existing library in
+DEPS, please ask the infrastructure team before committing the change.
+
+## Setting up ignore
+
+If your code is synced via DEPS, you should add the new directory to Chromium's
+`.gitignore`. This is necessary because Chromium's main git repository already
+contains
+[src/third_party](http://src.chromium.org/viewvc/chrome/trunk/src/third_party/)
+and the project synced via DEPS is nested inside of it and its files regarded as
+untracked. That is, anyone running `git status` from `src/` would see a clutter.
+Your project's files are tracked by your repository, not Chromium's, so make
+sure the directory is listed in Chromium's `.gitignore`.
+
+## Get a Review
+
+All third party additions and substantive changes like re-licensing need the
+following sign-offs. Some of these are accessible to Googlers only. Non-Googlers
+can email one of the people in third_party/OWNERS for help.
+
+* Chrome Eng Review. Googlers should see go/chrome-eng-review
+* open-source-third-party-reviews@google.com (ping the list with relevant
+  details and a link to the CL).
+* security@chromium.org (ping the list with relevant details and a link to the
+  CL).
+
+Third party code is a hot spot for security vulnerabilities. When adding a new
+package that could potentially carry security risk, make sure to highlight risk
+to security@chromium.org. You may be asked to add a README.security or, in
+dangerous cases, README.SECURITY.URGENTLY file. When you update your code, be
+mindful of security-related mailing lists for the project and relevant CVE to
+update your package.
+
+Subsequent changes don't require third-party-owners approval; you can modify the
+code as much as you want.
+
+## Ask the infrastructure team to add a git mirror for the dependency (or
+configure the git repo, if using googlesource.com)
+
+Before committing the DEPS, you need to ask the infra team to create a git
+mirror for your dependency. [Create a
+ticket](https://bugs.chromium.org/p/chromium/issues/entry?template=Infra-Git)
+for infra and ask the infra team.
+
+If you are using a git repo from googlesource.com then you must ensure that the
+repository is configured to give the build bots unlimited quota, or else the
+builders will fail to checkout with an "Over Quota" error. [Create a
+ticket](https://bugs.chromium.org/p/chromium/issues/entry?template=Infra-Git)
+for infra and ask the infra team what needs to be done. Note that you'll need
+unlimited quota for at least two role accounts. See the quota status of
+`boringssl` as an example.
+
+## Odds n' Ends
+
+### Handling `licenses_check (checklicenses.py)` failures
+
+This is needed for [Issue
+28291](http://code.google.com/p/chromium/issues/detail?id=28291): Pass the
+Ubuntu license check script:
+
+__If the failure looks like ...   ... the right action is to ... __
+
+* 'filename' has non-whitelisted license 'UNKNOWN'
+    * Ideally make the licensecheck.pl script recognize the license of that
+      file.  Often this requires __adding a license header__. Does it have
+      license header? If it's third party code, ask the upstream project to make
+      sure all their files have license headers.  If the license header is there
+      but is not recognized, we should try to patch licensecheck.pl.  If in
+      doubt please contact phajdan.jr@chromium.org
+* 'filename' has non-whitelisted license 'X' and X is BSD-compatible
+    * Add the license X to WHITELISTED_LICENSES in checklicenses.py . Make sure
+      to respect the OWNERS of that file. You must get an approval before
+      landing. CLs violating this requirement may be reverted.
+* 'filename' has non-whitelisted license 'X' and X is not BSD-compatible (i.e.
+  GPL)
+    * Do you really need to add those files? Chromium is BSD-licensed so the
+      resulting binaries can't use GPL code. Ideally we just shouldn't have
+      those files at all in the tree. If in doubt, please ask mal@chromium.org
+
+### Handling `webview_licenses.py` failures
+
+__If the failure looks like ...   ... the right action is to ... __
+
+* Missing license file
+    * Make sure that the license file is present. It should be called 'LICENSE',
+      or otherwise README.chromium file must point to it explicitly.
+* The following files contain a third-party license but are not in a listed
+  third-party directory...
+    * Check if it's a false positive (e.g. 'copyright' word used in a string
+      literal), if so, update
+      [src/tools/copyright_scanner/third_party_files_whitelist.txt](https://code.google.com/p/chromium/codesearch#chromium/src/tools/copyright_scanner/third_party_files_whitelist.txt)
+      file. Otherwise, please move the code into third_party.