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Mailbox is required in cc/paint for OOP-R canvas. This CL pulls mailbox
out of //gpu and into its own component.

This change also introduces a mailbox based DecodedDrawImage required
for OOPR-Canvas.

This is being done as part of the OOPR-Canvas2D project. For more info
about that project see the tracking bug here: crbug.com/1018894

Bug: 795884, 1031050
Change-Id: Ibf3930df8436a499f9b647b3a2da8cde65fa95be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2239292
Commit-Queue: Jonah Chin <jochin@microsoft.com>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779597}