Implement seekable() according to the MediaSource specification

Source/core/html/HTMLMediaElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 3334 matching lines @@
         m_playedTimeRanges = TimeRanges::create();
 
     return m_playedTimeRanges->copy();
 }
 
 PassRefPtrWillBeRawPtr<TimeRanges> HTMLMediaElement::seekable() const
 {
     if (!webMediaPlayer())
         return TimeRanges::create();
 
+    if (m_mediaSource)
+        return m_mediaSource->seekable();
+
     return TimeRanges::create(webMediaPlayer()->seekable());
 }
 
 bool HTMLMediaElement::potentiallyPlaying() const
 {
     // "pausedToBuffer" means the media engine's rate is 0, but only because it had to stop playing
     // when it ran out of buffered data. A movie is this state is "potentially playing", modulo the
     // checks in couldPlayIfEnoughData().
     bool pausedToBuffer = m_readyStateMaximum >= HAVE_FUTURE_DATA && m_readyState < HAVE_FUTURE_DATA;
     return (pausedToBuffer || m_readyState >= HAVE_FUTURE_DATA) && couldPlayIfEnoughData() && !isBlockedOnMediaController();
@@ skipping 729 matching lines @@
 
 #if ENABLE(WEB_AUDIO)
 void HTMLMediaElement::clearWeakMembers(Visitor* visitor)
 {
     if (!visitor->isAlive(m_audioSourceNode) && audioSourceProvider())
         audioSourceProvider()->setClient(nullptr);
 }
 #endif
 
 }