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In our current implementation, we have a Canvas2DImageBufferSurface that
inherits from ImageBufferSurface. The Canvas2DImageBufferSurface keeps
a RefPtr to Canvas2DLayerBridge, and pretty much all the functions in
Canvas2DImageBufferSurface directly calls the functions in Canvas2DLayerBridge.
The reason for this design is that Canvas2DLayerBridge is a RefCounted
class, so that the MailboxInfo could keep a pointer to Canvas2DLayerBridge.

This CL does a refactor in a way that makes Canvas2DLayerBridge directly
inherits from ImageBufferSurface.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Iceaff82d7561f1d064a0e2cc546f47098dd8e110
Reviewed-on: https://chromium-review.googlesource.com/576994Reviewed-by: Justin Novosad <junov@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491827}