Part of fixing  https://crbug.com/803021  will move us away from |local_streams_| and fix this issue at the same time. I could do the "blocked on" in either direction, but putting it as blocked on on the legacy API issue which is the primary issue being worked on.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/85000e8aea2a9b14b0618cf8351ffbcd5110e123

commit 85000e8aea2a9b14b0618cf8351ffbcd5110e123
Author: Henrik Boström <hbos@chromium.org>
Date: Mon Feb 12 10:52:40 2018

Redefine legacy addStream()/removeStream() on top of track-based APIs.

Implementation:
- addStream() is defined in terms of addTrack() and adding a listener
  of tracks being added/removed from the stream. The listener uses
  addTrack()/removeTrack().
- removeStream() is defined in terms of removeTrack() and removing the
  listener of tracks being added/removed from the stream.

This fixes:  https://crbug.com/738929 

Related bug fixes:
- By getting rid of local_streams_, this also fixes the issue where
  createDTMFSender() could only be called for tracks added with the
  legacy APIs.  https://crbug.com/806875 
- By adding code checking if a stream was the first/last stream
  added/removed, the chrome://webrtc-internals events related to
  adding or removing a stream are triggered by addTrack/removeTrack,
  fixing the bug where addTrack-streams did not appear in
  chrome://webrtc-internals.  https://crbug.com/801093 

Old code no longer exercised is removed. MockPeerConnectionImpl is
updated to work with track-based APIs. We should get rid of this class
in future CLs, it's a maintenance burden.

Design doc: https://docs.google.com/document/d/1Obbeg-B4_04twVctHdf7C7vkUQrGlLvOaV0C8J5_-Gs/edit?usp=sharing

Bug:  738929 ,  803021 ,  806875 ,  801093 
Change-Id: I77541a3dce180e1cf8b311cb7803047215b0af7b
Reviewed-on: https://chromium-review.googlesource.com/899346
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536052}
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/content/renderer/media/webrtc/mock_peer_connection_impl.cc
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/content/renderer/media/webrtc/mock_peer_connection_impl.h
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/content/renderer/media/webrtc/rtc_peer_connection_handler.cc
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/content/renderer/media/webrtc/rtc_peer_connection_handler.h
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/content/renderer/media/webrtc/rtc_peer_connection_handler_unittest.cc
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/content/renderer/media/webrtc/rtc_rtp_sender.cc
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/content/renderer/media/webrtc/rtc_rtp_sender.h
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/third_party/WebKit/LayoutTests/fast/peerconnection/RTCPeerConnection-AddRemoveTrack.html
[delete] https://crrev.com/b7e3a91cb94a9f47a8ebbef7a887d31069c18aac/third_party/WebKit/LayoutTests/fast/peerconnection/RTCPeerConnection-legacy-stream-APIs-expected.txt
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.h
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.idl
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/third_party/WebKit/Source/platform/testing/TestingPlatformSupportWithWebRTC.cpp
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/third_party/WebKit/Source/platform/testing/TestingPlatformSupportWithWebRTC.h
[modify] https://crrev.com/85000e8aea2a9b14b0618cf8351ffbcd5110e123/third_party/WebKit/public/platform/WebRTCPeerConnectionHandler.h