Add some logic to identify the active GPU in multiple-gpu situation in Linux

BUG=326783
TEST=gpu_unittests
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/94103006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240707 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240707