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 a0e191a77d535c552e0582eb0b385c33d232331b..dd5ebe802ec204783bb4f6b97041edbb36653ef7 100644
--- a/chrome/browser/ui/panels/panel_browser_window_cocoa.h
+++ b/chrome/browser/ui/panels/panel_browser_window_cocoa.h
@@ -51,6 +51,11 @@ 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);