Initial implementation of AudioTrack, AudioTrackList, VideoTrack, and VideoTrackList.

Source/core/html/track/AudioTrackList.cpp

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+#include "core/html/track/AudioTrackList.h"
+
+#include "core/html/HTMLMediaElement.h"
+#include "core/html/track/AudioTrack.h"
+
+namespace WebCore {
+
+AudioTrackList::~AudioTrackList()
+{
+}
+
+AudioTrackList::AudioTrackList(HTMLMediaElement* owner)
+    : TrackBaseList(owner)
+{
+    ScriptWrappable::init(this);
+}
+
+AudioTrack* AudioTrackList::anonymousIndexedGetter(unsigned index) const
+{
+    return static_cast<AudioTrack*>(getByIndex(index));
+}
+
+AudioTrack* AudioTrackList::getTrackById(const AtomicString& id) const
+{
+    return static_cast<AudioTrack*>(getById(id));
+}
+
+bool AudioTrackList::hasEnabledTrack() const
+{
+    for (unsigned i = 0; i < length(); ++i) {
+        if (anonymousIndexedGetter(i)->enabled())
+            return true;
+    }
+
+    return false;
+}
+
+const AtomicString& AudioTrackList::interfaceName() const
+{
+    return EventTargetNames::AudioTrackList;
+}
+
+ExecutionContext* AudioTrackList::executionContext() const
+{
+    return owner()->executionContext();

[fs, 2014/02/24 10:32:36]

I think owner() can end up being NULL here. I imaging a sequence like:

// Global reference to AudioTrackList:
trackList = mediaElement.audioTracks;
trackList.on... = <somehing>;

// Time elapses, mediaElement is GC'd, clearing owner.

trackList.on... = <something else>;

(see for instance issue 339592.)

Also, shouldn't the EventTarget stay alive while events are being delivered to
it? (Similar case, and absolutely very narrow/special-casey.)

[acolwell GONE FROM CHROMIUM, 2014/03/07 22:08:43]

Fixed. ActiveDOMObject takes care of this now.

+}
+
+void AudioTrackList::trackEnabled(const AtomicString& id, bool enabled)
+{
+    if (owner())
+        owner()->didEnabledAudioTrackChange(id, enabled);
+
+    scheduleChangeEvent();
+}
+
+}