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Both are per-webview objects.

RootFrameSink lives on the viz thread, and responsible for per-webview
objects that do not require access to the GPU. This is mostly classes
involved for sending begin frames.

RootFrameSinkProxy straddles UI and Viz thread and provides access to
RootFrameSink.

Bug: 805739
Change-Id: I4c7e31882d35eb3f772bd81b62a40a47c279de1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1749527
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686597}