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This is a mechanical refactor of RenderWidgetCompositor and
RenderWidget. At this point it removes references for
routing ID, and RenderWidget from RenderWidgetCompositor's
implementation. This is likely not the final refactor as I
am still in the process of detangling WebWidgetClient's
implementation from RenderWidget. Once that happens, I
might have some simplifications.

BUG=577321

Review URL: https://codereview.chromium.org/1587743002

Cr-Commit-Position: refs/heads/master@{#369383}