[Mojo Video Capture] Implement a VideoCaptureProvider that uses the video capture service

[Mojo Video Capture] Implement a VideoCaptureProvider that uses the video capture service

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

The new implementation is not yet hooked up to anything. This will be done in
the subsequent CL25.

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