Use a weak pointer when binding GuestViewBase::DidAttach.

components/guest_view/browser/guest_view_message_filter.h

Index: components/guest_view/browser/guest_view_message_filter.h
diff --git a/components/guest_view/browser/guest_view_message_filter.h b/components/guest_view/browser/guest_view_message_filter.h
index 7c271d8a98c1dd3fe414f768218284f8bc85f72c..1a24bf2cacdd29e52b76c47bf823c4d24c5bea6c 100644
--- a/components/guest_view/browser/guest_view_message_filter.h
+++ b/components/guest_view/browser/guest_view_message_filter.h
@@ -23,7 +23,6 @@ class BrowserContext;
 }
 
 namespace guest_view {
-class GuestViewBase;
 class GuestViewManager;
 
 // This class filters out incoming GuestView-specific IPC messages from the
@@ -75,8 +74,6 @@ class GuestViewMessageFilter : public content::BrowserMessageFilter {
   void OnViewCreated(int view_instance_id, const std::string& view_type);
   void OnViewGarbageCollected(int view_instance_id);
 
-  void WillAttachCallback(GuestViewBase* guest);
-
   DISALLOW_COPY_AND_ASSIGN(GuestViewMessageFilter);
 };