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As pointed out in https://crrev.com/c/1336218 it's easy to miss the
Start/Stop Compositor when changing the RenderWidget's swapped out
state, and it looks like they are separate when not done with a single
method call. For safety and clarity, move them into SetSwappedOut.

R=piman@chromium.org

Change-Id: Ic45d5d911255886835e283a5777a3923f63e1830
Bug: 896836
Reviewed-on: https://chromium-review.googlesource.com/c/1338492Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608580}