[remoting host] Move VideoTrackSource into header/source files.

This removes "Dummy" from the class name, and moves it to separate
header and source files. Eventually this class will have some
non-trivial implementation, with the move away from "internal source"
encoding.

Bug: 1192865
Change-Id: I1a08b158ce50e26871003210bee901e17982b2f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2787671
Commit-Queue: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866863}