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This moves much of the capturing logic to ThumbnailCaptureDriver which
doesn't depend on WebContents or video capture details. Instead, it
expects calls from its client on state changes and calls back into a
client interface to request, start, and stop capture.

Since the new class is testable, this also adds unit tests.

Bug: 1073141
Change-Id: I5e458ce4e7ab04f2091e1af4c76ad701d6eda0d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208547Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772842}