docs: start migrating android build instructions to markdown

docs/android_build_instructions.md

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+# Android Build Instructions
+
+[TOC]
+
+## Prerequisites
+
+A Linux build machine capable of building [Chrome for
+Linux](https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions_prerequisites.md).
+Other (Mac/Windows) platforms are not supported for Android.
+
+## Getting the code
+
+First, check out and install the [depot\_tools
+package](https://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up).
+
+Then, if you have no existing checkout, create your source directory and
+get the code:
+
+```shell
+mkdir ~/chromium && cd ~/chromium
+fetch --nohooks android # This will take 30 minutes on a fast connection
+```
+
+If you have an existing Linux checkout, you can add Android support by
+appending `target_os = ['android']` to your .gclient file (in the
+directory above src):
+
+```shell
+cat > .gclient <<EOF
+ solutions = [ ...existing stuff in here... ]
+ target_os = [ 'android' ] # Add this to get Android stuff checked out.
+EOF
+```
+
+Then run gclient sync to get the Android stuff checked out:
+
+```shell
+gclient sync
+```
+
+## (Optional) Check out LKGR
+
+If you want a single build of Chromium in a known good state, sync to
+the LKGR ("last known good revision"). You can find it
+[here](http://chromium-status.appspot.com/lkgr), and the last 100
+[here](http://chromium-status.appspot.com/revisions). Run:
+
+```shell
+gclient sync --nohooks -r <lkgr-sha1>
+```
+
+This is not needed for a typical developer workflow; only for one-time
+builds of Chromium.
+
+## Configure your build
+
+Android builds can be run with GN or GYP, though GN incremental builds
+are the fastest option and GN will soon be the only supported option.
+They are both meta-build systems that generate nina files for the
+Android build. Both builds are regularly tested on the build waterfall.
+
+### Configure GYP (deprecated -- use GN instead)
+
+If you are using GYP, next to the .gclient file, create a a file called
+'chromium.gyp_env' with the following contents:
+
+```shell
+echo "{ 'GYP_DEFINES': 'OS=android target_arch=arm', }" > chromium.gyp_env
+```
+
+Note that "arm" is the default architecture and can be omitted. If
+building for x86 or MIPS devices, change `target_arch` to "ia32" or
+"mipsel".
+
+ **NOTE:** If you are using the `GYP_DEFINES` environment variable, it
+will override any settings in this file. Either clear it or set it to
+the values above before running `gclient runhooks`.