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This is needed because the RenderThread in the renderer process owns a reference to the RenderWidget, and that reference is only released when the RenderWidgetHost sends ViewMsg_Close, which happens on RenderWidgetHost::Shutdown.

BUG=464764

Review URL: https://codereview.chromium.org/1052223008

Cr-Commit-Position: refs/heads/master@{#327331}