[Video Capture Service] Add multi-client support: Add VideoSourceProvider

First step towards adding multi-client support to the video capture
service. This CL adds new interfaces VideoSourceProvider and VideoSource,
which are the multi-client equivalent of DeviceFactory and Device from
the single-client case. It adds implementation for VideoSourceProvider
that internally uses a DeviceFactory.

Design Doc: https://docs.google.com/document/d/1mYnsZfLBRmbsDpUtfb6C7dzhfw2Kcxg_-uiG_6MnWVQ/edit?usp=sharing
Test: services_unittests --gtest_filter=MockVideoCaptureDeviceSharedAccessTest.*
Bug: 783442

Change-Id: Ic3142542d4c2f4be2a7ffb500def1f9407be1e64
Reviewed-on: https://chromium-review.googlesource.com/c/1262401
Commit-Queue: Christian Fremerey <[email protected]>
Reviewed-by: Luke Sorenson <[email protected]>
Reviewed-by: Mustafa Emre Acer <[email protected]>
Cr-Commit-Position: refs/heads/master@{#622533}
18 files changed