Create gn arg templates for use with blimp builds. Update build doc.

blimp/docs/build.md

 # Using GN
 Blimp only supports building using [GN](../../tools/gn/README.md). A quick
 overview over how to use GN can be found in the GN
 [quick start guide](../../tools/gn/docs/quick_start.md).
 
 There are three different build configurations depending on what you want to
 build:
 
 ## Android client
 
 Create an out-directory and set the GN args:
 
 ```bash
 mkdir -p out-android/Debug
-gn args out-android/Debug
-```
-
-This will bring up an editor, where you can type in the following:
-
-```bash
-target_os = "android"
-is_debug = true
-is_clang = true
-is_component_build = true
-symbol_level = 1  # Use -g1 instead of -g2
-use_goma = true
+echo "import(\"//build/args/blimp_client.gn\")" > out-android/Debug/args.gn
+gn gen out-android/Debug
 ```
 
 To build:
 
 ```bash
 ninja -C out-android/Debug blimp
 ```
 
 You can also build and install incremental APK like this:
 
 ```bash
 ninja -C out-android/Debug blimp blimp_apk_incremental &&
     out-android/Debug/bin/install_blimp_apk_incremental
 ```
 
-## Engine inside a Docker container
-
-Create another out-directory and set the GN args. Note, when building to run
-inside a [Docker container](container.md) you'll need to set the target_os to "chromeos":
-
+To add your own build preferences
 ```bash
-mkdir -p out-chromeos/Debug
-gn args out-chromeos/Debug
+gn args out-android/Debug
 ```
 
-This will bring an editor, where you can type in the following:
-
-```bash
-target_os = "chromeos"
-is_debug = true
-is_clang = true
-symbol_level = 1  # Use -g1 instead of -g2
-use_goma = true
-use_aura = true
-use_ozone = true
-use_alsa = false
-use_pulseaudio = false
-```
-
-To build:
-
-```bash
-ninja -C out-chromeos/Debug blimp
-```
-
-## "Bare" engine, no Docker container
+## Engine
 
 Create another out-directory and set the GN args:
 
 ```bash
 mkdir -p out-linux/Debug
-gn args out-linux/Debug
-```
-
-This will bring an editor, where you can type in the following:
-
-```bash
-is_debug = true
-is_clang = true
-is_component_build = true
-symbol_level = 1  # Use -g1 instead of -g2
-use_goma = true
-use_aura = true
-use_ozone = true
+echo "import(\"//build/args/blimp_engine.gn\")" > out-linux/Debug/args.gn
+gn gen out-linux/Debug
 ```
 
 To build:
 
 ```bash
 ninja -C out-linux/Debug blimp
 ```
+
+To add your own build preferences
+```bash
+gn args out-android/Debug
+```