diff --git a/dom/media/mediasource/MediaSourceDecoder.cpp b/dom/media/mediasource/MediaSourceDecoder.cpp
index 56d5c951833..96e3d696425 100644
--- a/dom/media/mediasource/MediaSourceDecoder.cpp
+++ b/dom/media/mediasource/MediaSourceDecoder.cpp
@@ -309,4 +309,11 @@ MediaSourceDecoder::IsActiveReader(MediaDecoderReader* aReader)
   return mReader->IsActiveReader(aReader);
 }
 
+double
+MediaSourceDecoder::GetDuration()
+{
+  ReentrantMonitorAutoEnter mon(GetReentrantMonitor());
+  return mMediaSourceDuration;
+}
+
 } // namespace mozilla
diff --git a/dom/media/mediasource/MediaSourceDecoder.h b/dom/media/mediasource/MediaSourceDecoder.h
index 968a31f1cb8..3d7b8bf016c 100644
--- a/dom/media/mediasource/MediaSourceDecoder.h
+++ b/dom/media/mediasource/MediaSourceDecoder.h
@@ -56,6 +56,9 @@ public:
   void Ended();
   bool IsExpectingMoreData() MOZ_OVERRIDE;
 
+  // Return the duration of the video in seconds.
+  virtual double GetDuration() MOZ_OVERRIDE;
+
   void SetInitialDuration(int64_t aDuration);
   void SetMediaSourceDuration(double aDuration, MSRangeRemovalAction aAction);
   double GetMediaSourceDuration();