Use event init dictionary in core/html/track

Source/core/html/track/TrackEvent.cpp

Index: Source/core/html/track/TrackEvent.cpp
diff --git a/Source/core/html/track/TrackEvent.cpp b/Source/core/html/track/TrackEvent.cpp
index 293a9478f924966479334890e35c1cf180cf2ff9..6ff6ea61564a99f3b50227b6eac1080553ceda0c 100644
--- a/Source/core/html/track/TrackEvent.cpp
+++ b/Source/core/html/track/TrackEvent.cpp
@@ -33,19 +33,20 @@
 
 namespace blink {
 
-TrackEventInit::TrackEventInit()
-{
-}
-
-
 TrackEvent::TrackEvent()
 {
 }
 
 TrackEvent::TrackEvent(const AtomicString& type, const TrackEventInit& initializer)
     : Event(type, initializer)
-    , m_track(initializer.track)
 {
+    const VideoTrackOrAudioTrackOrTextTrack& track = initializer.track();
+    if (track.isVideoTrack())
+        m_track = track.getAsVideoTrack();
+    if (track.isAudioTrack())

[philipj_slow, 2014/12/10 08:22:48]

I'm guessing the compiler can't figure out that these are mutually exclusive by
itself, so use else if here and add ASSERT_UNREACHED at the end?

[bashi, 2014/12/10 08:45:56]

Done. The compiler checks the type of the given value, but it can't figure out
they are subclass of TrackBase, so these conditions are needed.

...And I should have checked hasTrack() before using track().

+        m_track = track.getAsAudioTrack();
+    if (track.isTextTrack())
+        m_track = track.getAsTextTrack();
 }
 
 TrackEvent::~TrackEvent()