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This CL is a refactor with no intended behavior change.

The member was already there. This CL simply exposes it and starts using it,
rather than calling RenderWidget::compositor_deps(), since |render_widget_| may
be nullptr in the future.

Bug: 912193
Change-Id: Ib36ee51473100f2cee8fc685f6817647a1c6fc08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829675
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701651}