Add gclient hook to download SDK extras on bots.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 6fe932dbcc62b48cafdde3aee18b9c2cff508d8d..a9e04f42ccad066c1bcd45d5b17dc549a5afe7c9 100644
--- a/DEPS
+++ b/DEPS
@@ -550,6 +550,15 @@ hooks = [
     ],
   },
   {
+    # This downloads SDK extras and puts them in the
+    # third_party/android_tools/sdk/extras directory on the bots. Developers
+    # need to manually install these packages and accept the ToS.
+    'name': 'sdkextras',
+    'pattern': '.',
+    'action': ['python', 'src/build/download_sdk_extras.py',
+               'google_google_play_services_21.0.0'],

[navabi, 2014/11/21 09:40:29]

Added this as the versioning scheme. In the google_google_play_services_21.0.0
bucket there is the google/google_play_services/* directory for revision 21.0.0.

This way new revisions can be added to the google storage bucket (e.g.
gs://chrome-sdk-extras/google_google_play_services_22.0.0), and then the "roll"
would happen by changing this command line argument in this DEPS file.

This also allows adding other packages as the command line arguments for
download_sdk_extras.py takes a list of buckets.

+  },
+  {
     # Downloads the Debian Wheezy sysroot to chrome/installer/linux if needed.
     # This sysroot updates at about the same rate that the chrome build deps
     # change. This script is a no-op except for linux users who are doing