Remove disable-gpu-vsync about:flags entry

This entry completely breaks Chrome and has for a while.

BUG=303293
R=jbauman@chromium.org

Review URL: https://codereview.chromium.org/25814003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226626 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226626