SurfaceView: Wait to gather transparent region.

With the introduction of surfaceRedrawNeededAsync we may
be asked to gather the transparent region ahead of the SurfaceView
having been drawn.

Bug: 38324871
Test: Launch Chrome Canary a lot! No Flickers.
Change-Id: I35f09a1bb8316895fa704b10c912e64a8920bd90
(cherry picked from commit 3ca12be7)