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This patch sets the device scale factor on cc's root transform node,
fixing several layout tests. This approach is different from how
cc builds property trees (see post_local_scale_factor in
property_tree_builder's AddTransformNodeIfNeeded) where a post_local
transform is used, but aligns closer to blink's approach of using a
single transform per transform node.

This approach is temporary and will need to be refactored when we
begin supporting viewport transforms such as pinch-zoom.

BUG=642240
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2318933006
Cr-Commit-Position: refs/heads/master@{#417394}