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This sets up the framework for a LayerTreeHostRemote that implements
the LayerTreeHost API when the compositor is running across a network
boundary.

This change only sets the framework for running/scheduling main frame
updates and pushing the serialized state using the
CompositorProtoStateSink. Subsequent patches will add state
serialization.

BUG=648442
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2362073002
Cr-Commit-Position: refs/heads/master@{#422555}