Implement seekable() according to the MediaSource specification

Source/core/html/HTMLMediaSource.h

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 49 matching lines @@
     // 'closed' state, and returns true to indicate attachment success.
     // Reattachment allowed by first calling close() (even if already in 'closed').
     // Once attached, the source uses the element to synchronously service some
     // API operations like duration change that may need to initiate seek.
     virtual bool attachToElement(HTMLMediaElement*) = 0;
     virtual void setWebMediaSourceAndOpen(PassOwnPtr<blink::WebMediaSource>) = 0;
     virtual void close() = 0;
     virtual bool isClosed() const = 0;
     virtual double duration() const = 0;
     virtual PassRefPtrWillBeRawPtr<TimeRanges> buffered() const = 0;
+    virtual PassRefPtrWillBeRawPtr<TimeRanges> seekable() const = 0;
 #if !ENABLE(OILPAN)
     virtual void refHTMLMediaSource() = 0;
     virtual void derefHTMLMediaSource() = 0;
 #endif
 
     // URLRegistrable
     virtual URLRegistry& registry() const override { return *s_registry; }
 
 private:
     static URLRegistry* s_registry;
 };
 
 }
 
 #endif