Generate and assign media track ids in demuxers.

content/renderer/media/webmediaplayer_ms_unittest.cc

Index: content/renderer/media/webmediaplayer_ms_unittest.cc
diff --git a/content/renderer/media/webmediaplayer_ms_unittest.cc b/content/renderer/media/webmediaplayer_ms_unittest.cc
index 4ea16bfa4f63eaa7ba1414d9b0c7b05b909ac1b3..c5d80c03db7fb47a13952a1c721a153b8ce2bb4d 100644
--- a/content/renderer/media/webmediaplayer_ms_unittest.cc
+++ b/content/renderer/media/webmediaplayer_ms_unittest.cc
@@ -398,7 +398,7 @@ class WebMediaPlayerMSTest
                                                const blink::WebString& label,
                                                const blink::WebString& language,
                                                bool enabled) override {
-    return 0;
+    return blink::WebMediaPlayer::TrackId();
   }
   void removeAudioTrack(blink::WebMediaPlayer::TrackId) override {}
   blink::WebMediaPlayer::TrackId addVideoTrack(const blink::WebString& id,
@@ -406,7 +406,7 @@ class WebMediaPlayerMSTest
                                                const blink::WebString& label,
                                                const blink::WebString& language,
                                                bool selected) override {
-    return 0;
+    return blink::WebMediaPlayer::TrackId();
   }
   void removeVideoTrack(blink::WebMediaPlayer::TrackId) override {}
   void addTextTrack(blink::WebInbandTextTrack*) override {}