Clean up SyncCompositorFactory context creation
Separate out context creation that does not need surface or service
into separate CreateOffscreenContext call.
Do not create GLSurfaceStubs on client side, and pass in NULL window
so the context creates GLSurfaceStubs on service side.
Clean up share context code since compositor does not need an offscreen
context anymore.
BUG=370566
Review URL: https://codereview.chromium.org/
271763002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269618 0039d316-1c4b-4281-b951-d872f2087c98