Reland: Implement correct Panel closing sequence for Mac.

chrome/browser/ui/panels/panel_browser_window_cocoa.h

Index: chrome/browser/ui/panels/panel_browser_window_cocoa.h
diff --git a/chrome/browser/ui/panels/panel_browser_window_cocoa.h b/chrome/browser/ui/panels/panel_browser_window_cocoa.h
index 7cb1fc00b9fe2e3eb4ac9ea9ad024f019740b533..0328b0641ec9e93c4be7960e48f441fee89c052a 100644
--- a/chrome/browser/ui/panels/panel_browser_window_cocoa.h
+++ b/chrome/browser/ui/panels/panel_browser_window_cocoa.h
@@ -56,6 +56,11 @@ class PanelBrowserWindowCocoa : public NativePanel {
   Panel* panel() { return panel_.get(); }
   Browser* browser() const { return browser_.get(); }
 
+  // Callback from PanelWindowControllerCocoa that native window was actually
+  // closed. The window may not close right away because of onbeforeunload
+  // handlers.
+  void didCloseNativeWindow();
+
  private:
   friend class PanelBrowserWindowCocoaTest;
   FRIEND_TEST_ALL_PREFIXES(PanelBrowserWindowCocoaTest, CreateClose);