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For the following case:
<div style="background: black; height: 500px; overflow: scroll">
  <div style="background: rgba(255, 255, 255, 0.1); height: 10000px">Text</div>
</div>
previously we chose rgba(255, 255, 255, 0.1) as the layer's background
color, causing white flash when the color was used for checkerboarding.

Now blend overlapping layers of background colors.

Bug: 1147379
Change-Id: I3badbbf060265fa1d1d31081a4882837fef408e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533306
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827330}