GitLab

In this CL:
- On Browser: VCHost is not a BrowserMessageFilter nor
a  BrowserAssociatedInterface, and instead works like a
normal service implementation, Create()d on request and
hanging from the render process host.

-On renderer:
 - VideoCaptureMessageFilter, which was specific for
IPC, is removed altogether.
 - VCImpl (the "client"), does not need the IPC, and
instead gets an InterfacePtr to the remote VideoCaptureHost,
and uses it.

All that allows for a few simplifications around like, e.g.
VCImpl doesn't need to keep track of  |clients_pending_on_filter_|
nor of IO thread explicitly (using a ThreadChecker instead),
and also I cleaned up a bit more the unittests.

BUG=651897

Review-Url: https://chromiumcodereview.appspot.com/2430313007
Cr-Commit-Position: refs/heads/master@{#426903}