Send FrameSwapMessageQueue's messages in QueueMessageSwapPromise

In order to simplify RendererCompositorFrameSink and make it more similar
to ClientCompositorFrameSink, it's better to send swap messages in a swap
promise as opposed to in the frame sink.

BUG=726485

Review-Url: https://codereview.chromium.org/2899073006
Cr-Commit-Position: refs/heads/master@{#474865}
7 files changed