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These fields are only ever set for content layers such as canvas
and video. However, such uses don't make sense any more, because the
"parent" concept originally had to do with a parent layer, which is
no longer a concept in layer list mode.

Bug: 1008483

Change-Id: I053f9048e5a152f84c4df761f1b1500a5a0019c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129893Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755365}