Fix crash in renderer initialization when media pipeline is stopped

media/base/demuxer_stream_provider.h

 // 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.
 
 #ifndef MEDIA_BASE_DEMUXER_STREAM_PROVIDER_H_
 #define MEDIA_BASE_DEMUXER_STREAM_PROVIDER_H_
 
 #include "base/macros.h"
 #include "media/base/demuxer_stream.h"
 #include "media/base/media_export.h"
@@ skipping 21 matching lines @@
     URL,     // Indicates GetUrl() should be used
   };
 
   DemuxerStreamProvider();
   virtual ~DemuxerStreamProvider();
 
   // For Type::STREAM:
   //   Returns the first stream of the given stream type (which is not allowed
   //   to be DemuxerStream::TEXT), or NULL if that type of stream is not
   //   present.
+  //   NOTE: Once a DemuxerStream pointer is returned from GetStream it is
+  //   guaranteed to stay valid for as long as the Demuxer/DemuxerStreamProvider
+  //   is alive. But make no assumption that once GetStream returned a non-null
+  //   pointer for some stream type then all subsequent calls will also return
+  //   non-null pointer for the same stream type. In MSE Javascript code can
+  //   remove SourceBuffer from a MediaSource at any point and this will make
+  //   some previously existing streams inaccessible/unavailable.
   // Other types:
   //   Should not be called.
   virtual DemuxerStream* GetStream(DemuxerStream::Type type) = 0;
 
   // For Type::URL:
   //   Returns the URL parameters of the media to play. Empty URLs are legal,
   //   and should be handled appropriately by the caller.
   // Other types:
   //   Should not be called.
   virtual MediaUrlParams GetMediaUrlParams() const;
 
   virtual DemuxerStreamProvider::Type GetType() const;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(DemuxerStreamProvider);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_DEMUXER_STREAM_PROVIDER_H_