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Moves/Refactors a lot of test infrastructure that can be re-used in an
upcoming change to also test the new browser window capture impl:

ContentCaptureDeviceBrowserTestBase: A common base class that sets up a
content shell navigated to a test page, whose content can also be
changed as test procedures require.

FakeVideoCaptureStack: A simple representation of the entire downstream
video capture stack that just takes the screen-captured video frames an
stores them in a queue for later examination by the tests.

FrameTestUtil: Math/Color utilities for analyzing the content in the
captured video frames.

Bug: 806366
Change-Id: I899db11043944ea5a1206a58b2593a317222fdab
Reviewed-on: https://chromium-review.googlesource.com/1006072
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Xiangjun Zhang <xjz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558034}