Change RtcRTPSender to operate over MediaStreamComponent

... instead of WebMediaStreamTrack.

This is part of the effort to move as much public headers out of the
Blink platform topmost public directory (//third_party/blink/public/platform/).

BUG=704136
R=guidou@chromium.org

Change-Id: I78ecf75cfe602de5b11962ece2a05d11d90f088d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2273677Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#783912}

third_party/blink/renderer/modules/peerconnection/rtc_rtp_sender.cc

@@ -429,14 +429,14 @@ ScriptPromise RTCRtpSender::replaceTrack(ScriptState* script_state,
                                            "The peer connection is closed."));
     return promise;
   }
-  WebMediaStreamTrack web_track;
+  MediaStreamComponent* component = nullptr;
   if (with_track) {
     pc_->RegisterTrack(with_track);
-    web_track = with_track->Component();
+    component = with_track->Component();
   }
   ReplaceTrackRequest* request =
       MakeGarbageCollected<ReplaceTrackRequest>(this, with_track, resolver);
-  sender_->ReplaceTrack(web_track, request);
+  sender_->ReplaceTrack(component, request);
   return promise;
 }