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DirectCompositionSurfaceWin remains as the host of the DCLayerTree,
while DirectCompositionChildSurfaceWin handles drawing to the
IDCompositionSurface itself. This will allow us to have more Surfaces
in the layer tree in the future.

BUG=726957
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2926763002
Cr-Commit-Position: refs/heads/master@{#478794}