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This CL is part of the Mojo Video Capture work.
Design doc for dev work [1].

Before this CL, the following situation could happen:
1. VideoCaptureManager calls ServiceVideoCaptureProvider::GetDeviceInfosAsync(
     callback).
2. ServiceVideoCaptureProvider makes a request to the service and waits for an
   answer
3. The service process crashes (or otherwise loses connection)
=> |callback| is never run

This CL also adds a test for the scenario described above and a fix in the
production code. The approach for the fix is to wrap the callback in a media::ScopedCallbackRunner to makes sure that  it always gets run. This change required moving some of the Callback types to OnceCallback, which is work we
want to do anyway.

Bug: 584797

[1] https://docs.google.com/a/chromium.org/document/d/1Qw7rw1AJy0QHXjha36jZNiEuxsxWslJ_X-zpOhijvI8/edit?usp=sharing

Change-Id: Id60b6ffd04de4f7118218099cc1ccd437744d5fb
Reviewed-on: https://chromium-review.googlesource.com/575786
Commit-Queue: Christian Fremerey <chfremer@chromium.org>
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488433}