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This type is binary compatible with skia.mojom.Bitmap and only serves
to specify a difference in the generated bindings. Instead of copying
the pixels out of shared memory into a heap allocated buffer retained
by the SkBitmap, the bindings will allow the SkBitmap to simply retain
the given shared memory buffer. This avoids a potentially large copy.

This change leverages SkBitmap's support for having its pixel backing
managed externally via SkPixelRef. The BigBuffer received over IPC is
stored in a new SkPixelRef subclass. The outcome of this is that if
the BigBuffer was using shared memory then the SkBitmap will be able
to reference that memory directly, thus avoiding a copy of the pixels.

Finally, the Lacros code for screen capture is updated to use this new
type to eliminate a large buffer copy.

Bug: 1094460
Change-Id: I9249ad40ced16c8cbe42bbbe5d11d9820cc75d7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450838Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Darin Fisher <darin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815217}