Build support for WebRTC Chromium tests on Android.

In addition to the native APK tests for WebRTC, we
want to build and test the content_browsertests test
for each revision of WebRTC in our Chromium WebRTC FYI
waterfall.
This CL adds the needed configurations for that, that are used
by the buildbots in: https://codereview.chromium.org/26702005/

NOTRY=True
TEST=presubmit
BUG=305749

Review URL: https://codereview.chromium.org/26738003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229758 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229758