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Make the WebRtcMediaStreamAdapter, which represent local streams,
use the WebRtcMediaStreamTrackMap and AdapterRef classes for the
handling of initializing, getting and uninitializing local tracks.

WebRtcMediaStreamTrackAdapter is updated to allow local tracks from
non-local sources (removed DCHECKs) which happens when redirecting
remote sources as local streams.

This is one step closer to decoupling streams and tracks.

The same thing will be done in a follow-up CL for the remote streams
case, RemoteMediaStreamImpl.

BUG=705901, 700916

Review-Url: https://codereview.chromium.org/2897603004
Cr-Commit-Position: refs/heads/master@{#478592}