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 73537cbe28f1eca3def936bda18f4260f1f03b3f..fe89ca8e5b53068f4195c8fe4f3fc6937724e108 100644
--- a/media/blink/multibuffer_data_source.h
+++ b/media/blink/multibuffer_data_source.h
@@ -165,7 +165,8 @@ 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();