Support for onbufferedamountlow

Original review at https://webrtc-codereview.appspot.com/54679004/

BUG=https://code.google.com/p/chromium/issues/detail?id=496700

Review URL: https://codereview.webrtc.org/1207613006

Cr-Commit-Position: refs/heads/master@{#9527}

talk/app/webrtc/datachannelinterface.h

diff --git a/talk/app/webrtc/datachannelinterface.h b/talk/app/webrtc/datachannelinterface.h
index 6312262..90573eb 100644
--- a/talk/app/webrtc/datachannelinterface.h
+++ b/talk/app/webrtc/datachannelinterface.h
@@ -91,6 +91,8 @@
   virtual void OnStateChange() = 0;
   //  A data buffer was successfully received.
   virtual void OnMessage(const DataBuffer& buffer) = 0;
+  // The data channel's buffered_amount has changed.
+  virtual void OnBufferedAmountChange(uint64 previous_amount){};
 
  protected:
   virtual ~DataChannelObserver() {}