Improve comments on WebContentsObserver RenderFrame* methods and rename.

content/public/browser/web_contents.h

Index: content/public/browser/web_contents.h
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index 49ea4cde75b80a1fb743ad92a9b429d356f05365..879e8d8513c03fa6ccb621249098bb66873a0fab 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -192,6 +192,10 @@ class WebContents : public PageNavigator,
   virtual RenderFrameHost* GetFocusedFrame() = 0;
 
   // Calls |on_frame| for each frame in the currently active view.
+  // Note: The RenderFrameHost parameter is not guaranteed to have a live
+  // RenderFrame counterpart in the renderer process. Callbacks should check
+  // IsRenderFrameLive, as sending IPC messages to it in this case will fail
+  // silently.
   virtual void ForEachFrame(
       const base::Callback<void(RenderFrameHost*)>& on_frame) = 0;