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This refactors MailboxToSurfaceBridge a bit based on existing callers.
A follow-up change will merge GenMailboxCHROMIUM and
ProduceTextureDirectCHROMIUM.

DestroyMailboxTexture was dead code, so removed. We can resurrect in a
later CL if needed.

Bug: 847674
Change-Id: I8fa52a4b7288955d3571c41bdf7b23f0ca4290f0
Reviewed-on: https://chromium-review.googlesource.com/1083333Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565114}