Improve timing of frame attach/detach (see #3664)

- Move frame attachment from RenderFrameCreated to
  DidCommitProvisionalLoad. This has a number of advantages:
  - Significantly reduces the frequency of disconnects by avoiding
    the GetInterface/DidCommitNavigation race condition.
  - Stops connecting temporary frames (created during cross-origin
    navigation), making callback behavior more consistent.
- Split frame detach and destruction notifications into separate
  callbacks. OnFrameDetached now reflects a potentially recoverable
  state. Add a new OnFrameDestroyed callback for the unrecoverable
  destruction state.

libcef/renderer/frame_impl.h

@@ -81,7 +81,6 @@ class CefFrameImpl
                           CefRefPtr<CefProcessMessage> message) override;
 
   // Forwarded from CefRenderFrameObserver.
-  void OnAttached();
   void OnWasShown();
   void OnDidCommitProvisionalLoad();
   void OnDidFinishLoad();
@@ -105,7 +104,7 @@ class CefFrameImpl
                            LocalFrameAction action);
 
   enum class ConnectReason {
-    RENDER_FRAME_CREATED,
+    DID_COMMIT,
     WAS_SHOWN,
     RETRY,
   };