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This CL also refactor Reflector/BrowserCompositorOutputSurface as follows.

* BrowserCompositorOutputSurface ('s subclass) is responsible for creating Texture mailbox for mirroring
 - when reflector is set
 - or texture size is changed
* OutputSurface notifies reflector when mailbox is updated.

BUG=365662

Review URL: https://codereview.chromium.org/1123763003

Cr-Commit-Position: refs/heads/master@{#330042}