New class AdaptedVideoTrackSource.

BUG=webrtc:6353

Review-Url: https://codereview.webrtc.org/2328333002
Cr-Original-Commit-Position: refs/heads/master@{#14345}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6f5a6c318890bf544fc12fedd56db4bc29af69ef
8 files changed