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This allows ui::Layer to take scale factor into consideration when updating the cc::NinePatchLayer.

BUG=466950
R=danakj@chromium.org
R=oshima@chromium.org
TBR=sky@chromium.org
TEST=verify on HiDPI display as well as HiDPI + LowDPI extended mode that shadows are correct.

Review URL: https://codereview.chromium.org/1002393002

Cr-Commit-Position: refs/heads/master@{#320974}