It used to render to IOSurfaces and use setContents to directly give
them to the OSX compositor. This was worth the complexity only because
Chrome going to use that code path. This is no longer the case.

Instead we replace the implementation with one based on CAOpenGLLayer,
that basically gets a "draw" callback every frame. The complexity comes
from the fact that this callback is called from another thread, with
another CGL context.

BUG=angleproject:1233

Change-Id: I1878d0071d057e043e0bb9043d9849f50e00d023
Reviewed-on: https://chromium-review.googlesource.com/314031Reviewed-by: ccameron chromium <ccameron@chromium.org>
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>