WebUI: prevent WebContent to hold invalid pointer.

chrome/browser/ui/webui/constrained_web_dialog_delegate_base.h

Index: chrome/browser/ui/webui/constrained_web_dialog_delegate_base.h
diff --git a/chrome/browser/ui/webui/constrained_web_dialog_delegate_base.h b/chrome/browser/ui/webui/constrained_web_dialog_delegate_base.h
index 606d59d866e6dcdd6d12d9b4c8853732533ce482..cca2e8daef9abb2fa810efeeb3dd817356067f59 100644
--- a/chrome/browser/ui/webui/constrained_web_dialog_delegate_base.h
+++ b/chrome/browser/ui/webui/constrained_web_dialog_delegate_base.h
@@ -9,6 +9,7 @@
 
 #include "base/macros.h"
 #include "chrome/browser/ui/webui/constrained_web_dialog_ui.h"
+#include "content/public/browser/web_contents_observer.h"
 #include "ui/web_dialogs/web_dialog_ui.h"
 #include "ui/web_dialogs/web_dialog_web_contents_delegate.h"
 
@@ -23,6 +24,7 @@ class WebDialogDelegate;
 // Platform-agnostic base implementation of ConstrainedWebDialogDelegate.
 class ConstrainedWebDialogDelegateBase
     : public ConstrainedWebDialogDelegate,
+      public content::WebContentsObserver,
       public ui::WebDialogWebContentsDelegate {
  public:
   // |browser_context| and |delegate| must outlive |this| instance, whereas
@@ -45,6 +47,9 @@ class ConstrainedWebDialogDelegateBase
   gfx::Size GetMaximumSize() const override;
   gfx::Size GetPreferredSize() const override;
 
+  // WebContentsObserver interface
+  void WebContentsDestroyed() override;
+
   // WebDialogWebContentsDelegate interface.
   void HandleKeyboardEvent(
       content::WebContents* source,