Fixes issue that if we don't simulate a disabled

      vertex attrib 0 (as in the case of running on top
of OpenGL ES 2.0 or ANGLE) then we actually have
to disable it.

TEST=updated webgl conformance tests.
BUG=71960

Review URL: http://codereview.chromium.org/6456016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@74323 0039d316-1c4b-4281-b951-d872f2087c98