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There is minimal ambiguity for tree related classes to reference the
LayerTreeHostImpl as host_impl rather than layer_tree_host_impl, and
reasonably high value from shorter code.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ifb4e1c098ba8aa840aaa5c2ccf91e5b525a4be8b
Reviewed-on: https://chromium-review.googlesource.com/627389Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Commit-Queue: Walter Korman <wkorman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496489}