README.chromium hints that we didn't do this previously because of gmock using svn externals. But
maruel recently landed a patch that makes gclient --ignore-externals.
BUG=none
TEST=none
Review URL: http://codereview.chromium.org/6241018
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72866 0039d316-1c4b-4281-b951-d872f2087c98