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Make TaskQueueWebView an abstract interface. The old implementation
becomes TaskQueueSingleThread, with some cleanups. Add new TaskQueueViz
which implements code required for viz thread scheduling tasks to render
thread without ability to block on it at any time.

Add a kVizForWebView to switch between the two implementations, and use
to distinguish the code paths in the future.

Nothing uses TaskQueueViz in this CL yet, but the viz prototype has been
rebased onto this and works.

Bug: 805739
Change-Id: I0fe6c935fa7b1536700bd84950397ac756c4209e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729473
Commit-Queue: Bo <boliu@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683530}