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This CL ensures that device scale factor is plumbed alongside the viewport
size from commit time through pending and then the active trees.

In a subsequent CL, I will introduce diagnostic checks that verify that
if device scale factor and viewport change, then the LocalSurfaceId must
also change.

Bug: 672962
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ica059593c9890609deb76ad44e1e58e0ff30edbd
Reviewed-on: https://chromium-review.googlesource.com/1159115Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579948}