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This is a first step to separate exo::SurfaceFactoryOwner into
exo::CompositorFrameSink and exo::CompositorFrameSinkHolder
(WIP CL: https://codereview.chromium.org/2493223002)

BUG=659601
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2544103002
Cr-Commit-Position: refs/heads/master@{#436117}