Add gtest_filter to Android test execution for WebRTC

We're seeing a lot of red builds due to tests out of
our control for our Android builder running
content_browsertests in
http://build.chromium.org/p/chromium.webrtc.fyi/waterfall

By filtering out our WebRTC specific tests, we'll get
a better signal of if changes in WebRTC ToT is causing
trouble in Chrome on Android.

NOTRY=True
BUG=305749
TEST=Android trybots passing + 
built with a workspace setup as https://code.google.com/p/webrtc/issues/detail?id=1882#c11:
. build/android/envsetup.sh
GYP_DEFINES="$GYP_DEFINES enable_tests=1" android_gyp
ninja -C out/Debug android_builder_chromium_webrtc
build/android/buildbot/bb_run_bot.py --bot-id=webrtc-chromium-tests --testing

Verified the output and ran the produced command line locally:
build/android/test_runner.py gtest -s content_browsertests --verbose '--gtest-filter=Webrtc*:WebRTC*' --num_retries=1

Verified that only the WebRTC test were excuted.

Review URL: https://codereview.chromium.org/59593003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234467 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234467