[Render Frame Host] Add comment about storage.

Change-Id: If4178a717f159d7df8c37095ec02957a16ef2c41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437741Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813685}

content/public/browser/render_frame_host.h

@@ -85,6 +85,9 @@ class StoragePartition;
 class WebUI;
 
 // The interface provides a communication conduit with a frame in the renderer.
+// The preferred way to keep a reference to a RenderFrameHost is storing a
+// GlobalFrameRoutingId and using RenderFrameHost::FromID() when you need to
+// access it.
 class CONTENT_EXPORT RenderFrameHost : public IPC::Listener,
                                        public IPC::Sender {
  public: