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This CL is part of the Mojo Video Capture work. For the bigger picture,
see [1] CL24_part1.

Goal of CL24:
The interface content::VideoCaptureProvider currently has one implementation
called InProcessVideoCaptureProvider, which is essentially a factory for the
legacy in-process video capture stack. This CL adds a second implementation
called MojoServiceVideoCaptureProvider which is essentially a wrapper for
connecting to and communicating with the new video capture service.

Changes in part1:
* Add class MojoServiceVideoCaptureProvider
* Add skeletons for classes MojoServiceVideoCaptureDeviceLauncher and
  MojoServiceLaunchedVideoCaptureDevice
* Introduce a build flag enable_mojo_video_capture to prevent the new and
  incomplete code from going into builds.

BUG=584797
TEST=
  service_unittests --gtest_filter="*Video*"
  content_unittests --gtest_filter="*Video*"
  content_browsertests --gtest_filter="VideoCaptureBrowserTest.*"

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

Review-Url: https://codereview.chromium.org/2848973002
Cr-Commit-Position: refs/heads/master@{#469380}