MSE: Complete letting SourceBuffer begin to do initialization segment received algorithm

public/platform/WebSourceBuffer.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 30 matching lines @@
 public:
     enum AppendMode {
         AppendModeSegments,
         AppendModeSequence
     };
 
     virtual ~WebSourceBuffer() { }
 
     // This will only be called once and only with a non-null pointer to a
     // client whose ownership is not transferred to this WebSourceBuffer.
-    virtual void setClient(WebSourceBufferClient*)
-    {
-        // FIXME: Remove default implementation once Chromium impl lands.
-    }
+    virtual void setClient(WebSourceBufferClient*) = 0;
 
     virtual bool setMode(AppendMode) = 0;
     virtual WebTimeRanges buffered() = 0;
 
     // Appends data and runs the segment parser loop algorithm.
     // The algorithm may update |*timestampOffset| if |timestampOffset| is not null.
     virtual void append(const unsigned char* data, unsigned length, double* timestampOffset) = 0;
 
     virtual void abort() = 0;
     virtual void remove(double start, double end) = 0;
     virtual bool setTimestampOffset(double) = 0;
 
     // Set presentation timestamp for the start of append window.
     virtual void setAppendWindowStart(double) = 0;
 
     // Set presentation timestamp for the end of append window.
     virtual void setAppendWindowEnd(double) = 0;
 
     // After this method is called, this WebSourceBuffer should never use the
     // client pointer passed to setClient().
     virtual void removedFromMediaSource() = 0;
 };
 
 } // namespace blink
 
 #endif