AW docs: device/emulator setup instructions

No change to code, only docs.

This adds device/emulator setup instructions. This recommends the
emulator, as it's publicly available and easy to setup. For physical
devices, this points to an Android site hosting publicly available
images (and directs Googlers to internal instructions).

This also documents how to build AOSP, but recommends against doing so.
The intent is to update
https://www.chromium.org/developers/how-tos/build-instructions-android-webview
to point to this page, so I've included this for completeness.

R=ctzsm@chromium.org

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I668171e9ed76a3af36429fc9218e4b236b4dd085
Reviewed-on: https://chromium-review.googlesource.com/c/1490407Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636210}

android_webview/docs/device-setup.md

+# Device Setup for WebView development
+
+[TOC]
+
+As WebView is an Android system component (rather than just an app), WebView
+imposes additional requirements on the developer workflow. In particular,
+WebView requires a physical device or emulator with a `userdebug` or `eng`
+Android image.
+
+To build WebView, or [run WebView's automated tests](./test-instructions.md),
+you'll need to set up either an emulator or a physical device.
+
+## Emulator (easy way)
+
+*** promo
+Unless you have a hardware-specific bug, or need to use a pre-release Android
+version, a physical device is usually unnecessary. An `x86` emulator should be
+easier to setup.
+***
+
+You can generally follow chromium's [Android
+emulator](/docs/android_emulator.md) instructions. You should choose a **Google
+APIs** image. The AOSP-based image will also work, but imposes additional
+developer hurdles.
+
+
+## Physical device
+
+*** promo
+If you're a Googler, please see internal instructions
+[here](http://go/clank-webview/building-webview/device-setup).
+***
+
+External developers can obtain Nexus/Pixel images
+[here](https://source.android.com/setup/build/running.html).
+
+## Building AOSP yourself (hard way)
+
+*** note
+This takes significantly longer than the two previous methods, so please
+strongly consider one of the above first.
+***
+
+**Prerequisite:** a machine capable of [building
+Android](http://source.android.com/source/building.html).
+
+Clone an AOSP checkout, picking a branch supported for your device (you'll need
+a branch above 5.0.0) from the [list of
+branches](https://source.android.com/setup/start/build-numbers.html#source-code-tags-and-builds):
+
+```shell
+mkdir aosp/ && cd aosp/ && \
+  repo init -u https://android.googlesource.com/platform/manifest -b android-9.0.0_r33 && \
+  repo sync -c -j<number>
+```
+
+You can obtain binary drivers for Nexus/Pixel devices
+[here](https://developers.google.com/android/drivers). Drivers should match your
+device and branch. Extract and run the shell script:
+
+```shell
+# Change the filenames to match your device/branch (this uses "crosshatch" as an
+# example)
+tar -xvzf /path/to/qcom-crosshatch-pd1a.180720.030-bf86f269.tgz
+./extract-qcom-crosshatch.sh # Extracts to the vendor/ folder
+```
+
+You can build AOSP and flash your device with:
+
+```shell
+source build/envsetup.sh
+device="crosshatch" # Change this depending on your device hardware
+lunch aosp_${device}-userdebug
+make -j<number>
+
+# Flash to device
+adb reboot bootloader
+fastboot -w flashall
+```
+
+For more information, please defer to [official
+instructions](https://source.android.com/setup/build/downloading).
+
+## Why won't a user image work?
+
+`user` images have all of Android's security features turned on (and they can't
+be disabled). In particular, you won't be able to install a locally built
+WebView:
+
+* Most `user` images are `release-keys` signed, which means local WebView builds
+  can't install over the preinstalled standalone WebView. This blocks
+  development on L-M, since this is the only WebView provider.
+* On N+, although you _can install_ a locally compiled
+  `monochrome_{public_}apk`, this is not a valid WebView provider. Unlike on
+  `userdebug`/`eng` images, the WebView update service performs additional
+  signature checks on `user` images, only loading code that has been signed by
+  one of the expected signatures--as above, these keys are not available for
+  local builds.
+
+Both of the above are important security features: these protect users from
+running malware in the context of WebView (which runs inside the context of
+apps).

android_webview/docs/navbar.md

 # WebView docs (go/webview-docs)
 
 * [Home][home]
+* [Device Setup](/android_webview/docs/device-setup.md)
 * [Test instructions](/android_webview/docs/test-instructions.md)
 * [Commandline flags](/android_webview/docs/commandline-flags.md)
 * [Net debugging](/android_webview/docs/net-debugging.md)