Bug 1617033 - part3 : add comment to explicitly mention some functions should only be used after starting listener. r=chunmin

Differential Revision: https://phabricator.services.mozilla.com/D63916

dom/html/HTMLMediaElement.cpp

@@ -426,6 +426,11 @@ class HTMLMediaElement::MediaControlEventListener final
     mControlAgent = nullptr;
   }
 
+  bool IsStarted() const { return mState != ControlledMediaState::eStopped; }
+
+  /**
+   * Following methods should only be used after starting listener.
+   */
   void NotifyMediaStartedPlaying() {
     MOZ_ASSERT(NS_IsMainThread());
     MOZ_ASSERT(IsStarted());
@@ -474,8 +479,6 @@ class HTMLMediaElement::MediaControlEventListener final
     }
   }
 
-  bool IsStarted() const { return mState != ControlledMediaState::eStopped; }
-
  private:
   ~MediaControlEventListener() = default;