Fall back on browser language if not otherwise specified.

BUG=326712

Review URL: https://codereview.chromium.org/116503003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241388 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241388
Added CrOS-specific OAuth2 browser test.

Expanded FakeGaia class to cover all other auth related calls.

BUG=none
TEST=OAuth2Test.*
R=rogerta@chromium.org, xiyuan@chromium.org

Review URL: https://codereview.chromium.org/99863007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241389 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241389