Add docs for GN/Android

site/user/quick/gn.md

 GN
 =====
 
 [GN](https://chromium.googlesource.com/chromium/src/tools/gn/)
 is a new meta-build system originally designed to replace GYP in Chromium.
 
 You can build Skia using GN in a limited number of configurations.  We expect
 that as that limited number rises, GN will become the preferred, and then only,
 way to build Skia.
 
 Supported Features
 ----------
 
-    * Linux, Mac
+    * Linux, Mac, Android
     * Software and GL rendering
     * libskia.a, libskia.so
     * DM, nanobench
 
 Quickstart
 ----------
 
 Please check out Skia using the instructions in one of the other quick start
 guides.  We diverge where they'd first run some command with "gyp" in it.
 
 <!--?prettify lang=sh?-->
 
     # After gclient sync, run fetch-gn to make sure you have GN.
     gclient sync && bin/fetch-gn
 
     # Run GN to generate your build files.  Some examples.
     gn gen out/Debug
-    gn gen out/Release --args=is_debug=false
-    gn gen out/Clang --args='cc="clang" cxx="clang++"'
-    gn gen out/Shared --args=is_component_build=true
+    gn gen out/Release  --args='is_debug=false'
+    gn gen out/Clang    --args='cc="clang" cxx="clang++"'
+    gn gen out/Shared   --args='is_component_build=true'
+    gn gen out/Cached   --args='compiler_prefix="ccache"'
+    gn gen out/Stripped --args='extra_cflags="-g0"'
+    gn gen out/RTTI     --args='extra_cflags_cc="-frtti"'
 
     # Build
+    ninja -C out/Debug
     ninja -C out/Release
-    ninja -C out/Debug
     ninja -C out/Clang
     ninja -C out/Shared
+    ninja -C out/Cached
+    ninja -C out/Stripped
+    ninja -C out/RTTI
 
 From here everything is pretty much business as usual.
+
+Android
+-------
+
+To build Skia for Android you need an [Android
+NDK](https://developer.android.com/ndk/index.html).
+
+If you do not have an NDK and have access to CIPD, you
+can use one of these commands to fetch the NDK our bots use:
+
+<!--?prettify lang=sh?-->
+
+    python infra/bots/assets/android_ndk_linux/download.py  -t /tmp/ndk
+    python infra/bots/assets/android_ndk_darwin/download.py -t /tmp/ndk
+
+When generating your GN build files, pass the path to your `ndk` and your
+desired `target_cpu`:
+
+<!--?prettify lang=sh?-->
+
+    gn gen out/arm      --args='ndk="/tmp/ndk" target_cpu="arm"'
+    gn gen out/arm64    --args='ndk="/tmp/ndk" target_cpu="arm64"'
+    gn gen out/mips64el --args='ndk="/tmp/ndk" target_cpu="mips64el"'
+    gn gen out/mipsel   --args='ndk="/tmp/ndk" target_cpu="mipsel"'
+    gn gen out/x64      --args='ndk="/tmp/ndk" target_cpu="x64"'
+    gn gen out/x86      --args='ndk="/tmp/ndk" target_cpu="x86"'
+
+Other arguments like `is_debug` and `is_component_build` continue to work.