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This CL adds exception for VizCompositorThreadRunnerWebView to use base
sync primitives to allow blocking UI/Android Renderer thread on viz
thread for viz for webview implementation.

Bug: 805739
Change-Id: I25d6b7494d6b722f1f1d1c6faaff84b552279d06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911386Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714679}