Keep Compositor Offscreen context in sync compositor

This is used as the share context for video on main thread. Until we can
properly clean up the video context, just skip releasing the this
context to avoid issues during mode switch.

BUG=346376
NOTRY=true

Review URL: https://codereview.chromium.org/178523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253081 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253081