content: adds comment to RenderFrameCreated()

Hopefully this comment will help those trying to understand message
ordering. RenderFrameCreated() is a good place to send a message to the
renderer before loading starts.

BUG=none
TEST=none

Change-Id: Idd7e3d9b8c6e41daf6b1d50cd4088bd12132deae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2212286Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771080}

content/public/browser/web_contents_observer.h

@@ -80,8 +80,10 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener {
   // Frames and Views ----------------------------------------------------------
 
   // Called when a RenderFrame for |render_frame_host| is created in the
-  // renderer process. Use |RenderFrameDeleted| to listen for when this
-  // RenderFrame goes away.
+  // renderer process. This function is called before starting to load the
+  // document. Messages sent to the renderer from this function are guaranteed
+  // to arrive before any calls to RenderFrameObserver::DidClearWindowObject().
+  // Use |RenderFrameDeleted| to listen for when this RenderFrame goes away.
   virtual void RenderFrameCreated(RenderFrameHost* render_frame_host) {}
 
   // Called when a RenderFrame for |render_frame_host| is deleted or the