Move XInitThreads() from Compositor test suite to GLSurface init.

Any unit tests that use OSMesa GL need to call XInitThreads() first.
Currently that is just the compositor unittests, but when we move all
unit tests to use a real compositor with real GL contexts and stubbed
out drawing commands, this becomes all unit tests that use the
compositor. So move this call to GLSurface::InitializeOneOffForTests.

TBR=piman@chromium.org
BUG=270918

Review URL: https://codereview.chromium.org/174193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252415 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252415