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This simplifies the reasoning about when these are present, and will make
managing them with a layout manager slightly easier.

Bug: none
Change-Id: I5868dbb31bd4022010936611fe794185014e878d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857195Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706189}