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In the following case
 <div style="will-change: transform; width: 300px; height: 300px; background: blue">
   <div style="width: 250px; height: 250px; background: green"></div>
 </div>
we'll use green as the background color instead of blue. Though the
green background is smaller than the blue background, green affects
more area than blue.

Bug: 1143099
Change-Id: I5032c0448a827c8ddf9e97d460147227e8a0fed2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508292Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823731}