cros: Update linux-chromeos documentation

* Eliminate references to Windows
* Specify the binary to run
* Document GN args

Bug: none
Test: markdown preview
Change-Id: I29b3179b4b74289a35e53cf2cd2ff5658ef7df6e
Reviewed-on: https://chromium-review.googlesource.com/884321Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531729}

docs/chromeos_build_instructions.md

 # Chrome OS Build Instructions (Chromium OS on Linux)
 
-Chromium on Chromium OS is built from a mix of code sourced from Chromium
-on Linux and Chromium on Windows. Much of the user interface code is
-shared with Chromium on Windows.
+Chromium on Chromium OS uses Linux Chromium as a base, but adds a large number
+of features to the code. For example, the login UI, window manager and system UI
+are part of the Chromium code base and built into the chrome binary.
 
-If you make changes to Chromium on Windows, they may affect Chromium
-on Chromium OS. Fortunately to test the effects of your changes you
-don't need to build all of Chromium OS, you can just build Chromium for
-Chromium OS directly on Linux.
+Fortunately, most Chromium changes that affect Chromium OS can be built and
+tested on a Linux workstation. This build is called "linux-chromeos". In this
+configuration most system services (like the power manager, bluetooth daemon,
+etc.) are stubbed out. The entire system UI runs in a single X11 window on your
+desktop.
 
 First, follow the [normal Linux build
 instructions](https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions.md)
@@ -15,38 +16,52 @@ as usual to get a Chromium checkout.
 
 ## Building and running Chromium with Chromium OS UI on your local machine
 
-If you plan to test the Chromium build on your dev machine and not a
-Chromium OS device, run the following in your chromium checkout:
+Run the following in your chromium checkout:
 
     $ gn gen out/Default --args='target_os="chromeos"'
-    $ ninja -C out/Default -j32
+    $ autoninja -C out/Default chrome
+    $ out/Default/chrome
+
+Some additional options you may wish to set by passing in `--args` to `gn gen`
+or running `gn args out/Default`:
+
+    use_goma = true            # Googlers: Use build farm, compiles faster.
+    is_component_build = true  # Links faster.
+    is_debug = false           # Release build, runs faster.
+    dcheck_always_on = true    # Enables DCHECK despite release build.
+    enable_nacl = false        # Skips native client build, compiles faster.
+
+    # Set the following true to create a Chrome (instead of Chromium) build.
+    # This requires a src-internal checkout.
+    is_chrome_branded = false  # Adds internal features and branded art assets.
+    is_official_build = false  # Turns on many optimizations, slower build.
 
 NOTE: You may wish to replace 'Default' with something like 'Cros' if
 you switch back and forth between Linux and Chromium OS builds, or 'Debug'
-if you want to differentiate between Debug and Release builds (see below)
-or DebugCros or whatever you like.
-
-NOTE: If using goma increase -j32 (which controls parallelization) to something
-higher, 500 or 1000. Consider adding `-l20` which prevents ninja from starting
-jobs if the load average is too high - this prevents system hangs when using
--j1000 when goma is down.
+if you want to differentiate between Debug and Release builds (see below).
 
 See [GN Build Configuration](https://www.chromium.org/developers/gn-build-configuration)
 for more information about configuring your build.
 
-Some additional options you may wish to set by passing in `--args` to `gn gen`
-or running `gn args out/Default`:
+You can also build and run test targets like `unit_tests`, `browser_tests`, etc.
 
-    is_component_build = true
-    use_goma = true
-    is_debug = false  # Release build, significantly faster
-    dcheck_always_on = true  # Enable DCHECK (with is_debug = false)
+## Login notes
 
-    # Set the following true to create a Chrome (instead of Chromium) build.
-    is_official_build = false
-    is_chrome_branded = false
+By default this build signs in with a stub user. To specify a real user:
 
-## Notes
+*   For first run, add the following options to chrome's command line:
+    `--user-data-dir=/tmp/chrome --login-manager`
+*   Go through the out-of-the-box UX and sign in with a real Gmail account.
+*   For subsequent runs, add:
+    `--user-data-dir=/tmp/chrome --login-user=username@gmail.com`
+*   To run in guest mode instantly, add:
+    `--user-data-dir=/tmp/chrome --bwsi --incognito --login-user='$guest'
+    --login-profile=user`
+
+Signing in as a specific user is useful for debugging features like sync
+that require a logged in user.
+
+## Graphics notes
 
 The Chromium OS build requires a functioning GL so if you plan on
 testing it through Chromium Remote Desktop you might face drawing
@@ -59,22 +74,7 @@ problems (e.g. Aura window not painting anything). Possible remedies:
 To more closely match the UI used on devices, you can install fonts used
 by Chrome OS, such as Roboto, on your Linux distro.
 
-To specify a logged in user:
-
-*   For first run, add the following options to the command line:
-    `--user-data-dir=/tmp/chrome --login-manager`
-*   Go through the out-of-the-box UX and sign in as
-    **username@gmail.com**.
-*   For subsequent runs, add the following to the command line:
-    `--user-data-dir=/tmp/chrome --login-user=username@gmail.com`
-*   To run in guest mode instantly, you can run add the arguments:
-    `--user-data-dir=/tmp/chrome --bwsi --incognito --login-user='$guest'
-    --login-profile=user`
-
-Signing in as a specific user is useful for debugging features like sync
-that require a logged in user.
-
-## Compile Chromium for a Chromium OS device using the Chromium OS SDK
+## Compile Chromium for a Chromium OS device
 
-See [Building Chromium for a Chromium OS device](https://www.chromium.org/chromium-os/how-tos-and-troubleshooting/building-chromium-browser)
-for information about building and testing chromium for Chromium OS.
+See [Building Chromium for a Chromium OS device](https://chromium.googlesource.com/chromiumos/docs/+/master/simple_chrome_workflow.md)
+for information about building and testing Chromium for Chromium OS.