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TransformTree::CombineTransformsBetween is modified to compute the
combined transform between source and destination directly (without
walking the path) when the destination and all its ancestors are
flat. A unit test is added to validate the patch.

BUG=475636
TEST=passes all the unit tests along with the one added to validate the
patch. Telemetry test on poster circle page shows improvement in
performance. The draw_properties.tough_compositor:CDP_reduction percentage
improved from -287.82% to -149.36%.

Review URL: https://codereview.chromium.org/1096493002

Cr-Commit-Position: refs/heads/master@{#325503}