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This optimization reduces the damage rect size when the current damage rect
doesn't cover the video quad. For example, the video is paused or no video
change in this frame.

Because the overlay image is missing in gl composition, there might be
corruption if the old overlay needs to be redrawn in gl composition but the
damage rect doesn't cover it. Although sometimes it's needed to add the
overlay damage rect from the previous frame. It might increase the damage rect
than necessary when overlay is the same.

The unioned overlay quad size of the previous frame was always added to the
damage rect of the next frame before calling the overlay processor. This CL
optimizes the damage rect by handling the overlay damage rect in
DCLayerOverlayProcessor().

The other change of this CL is to move overlay and underlay validation code into
one function. This makes it easier to read and makes sure there is no validation
failures after the damage rect has changed.

BUG=868561

Change-Id: Ia66b950d5f010cb4b65288276cb9486e0d3d7709
Reviewed-on: https://chromium-review.googlesource.com/c/1472190
Commit-Queue: Maggie Chen <magchen@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634952}