WebUI: prevent WebContent to hold invalid pointer.

chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/webui/constrained_web_dialog_delegate_base.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/mac/scoped_nsobject.h"
 #include "base/macros.h"
@@ skipping 100 matching lines @@
   // ConstrainedWebDialogDelegate interface
   const WebDialogDelegate* GetWebDialogDelegate() const override {
     return impl_->GetWebDialogDelegate();
   }
   WebDialogDelegate* GetWebDialogDelegate() override {
     return impl_->GetWebDialogDelegate();
   }
   void OnDialogCloseFromWebUI() override {
     return impl_->OnDialogCloseFromWebUI();
   }
-  void ReleaseWebContentsOnDialogClose() override {
-    return impl_->ReleaseWebContentsOnDialogClose();
+  std::unique_ptr<content::WebContents> ReleaseWebContents() override {
+    return impl_->ReleaseWebContents();
   }
   gfx::NativeWindow GetNativeDialog() override { return window_; }
   WebContents* GetWebContents() override { return impl_->GetWebContents(); }
   gfx::Size GetMinimumSize() const override {
     return min_size_;
   }
   gfx::Size GetMaximumSize() const override {
     return max_size_;
   }
   gfx::Size GetPreferredSize() const override {
@@ skipping 120 matching lines @@
     const gfx::Size& max_size) {
   DCHECK(!min_size.IsEmpty());
   DCHECK(!max_size.IsEmpty());
   // Deleted when the dialog closes.
   ConstrainedWebDialogDelegateViewMac* constrained_delegate =
       new ConstrainedWebDialogDelegateViewMac(
           browser_context, delegate, web_contents,
           min_size, max_size);
   return constrained_delegate;
 }