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This is to avoid too low scale when animating scale above the
scrollbar layer.

This is not ideal. In the future we should move rasterization of
scrollbar layers to impl-side to better handle scales
(crbug.com/1009291).

Bug: 1133997
Change-Id: I3364754f530914816fe8526c627e6cf12e7942ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453436Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814681}