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This CL implements aura::WindowPortMus::CreateCompositorFrameSink().
Exo will use it in Mushrome and Mustash to create a frame sink
and use it to submit frames.

BUG=720600

Review-Url: https://codereview.chromium.org/2875753002
Cr-Original-Commit-Position: refs/heads/master@{#478287}
Committed: https://chromium.googlesource.com/chromium/src/+/810bd6f44ea3be0e7ff4522190cf2e4924769e17
Review-Url: https://codereview.chromium.org/2875753002
Cr-Commit-Position: refs/heads/master@{#478355}