Fix a timing bug in multibuffer.

media/blink/multibuffer_data_source.h

Index: media/blink/multibuffer_data_source.h
diff --git a/media/blink/multibuffer_data_source.h b/media/blink/multibuffer_data_source.h
index 5d3df792b0cc11e82c8148afa968998282128fce..eaaf94200655257856333634222260a0cc0a80f4 100644
--- a/media/blink/multibuffer_data_source.h
+++ b/media/blink/multibuffer_data_source.h
@@ -171,7 +171,8 @@ class MEDIA_BLINK_EXPORT MultibufferDataSource : public DataSource {
   // call downloading_cb_ if needed.
   // If |force_loading| is true, we call downloading_cb_ and tell it that
   // we are currently loading, regardless of what reader_->IsLoading() says.
-  void UpdateLoadingState(bool force_loading);
+  // Caller must hold |lock_|.
+  void UpdateLoadingState_Locked(bool force_loading);
 
   // Update |reader_|'s preload and buffer settings.
   void UpdateBufferSizes();