WebUI: prevent WebContent to hold invalid pointer.

chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm

Index: chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
diff --git a/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm b/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
index 9ef87e6b73aa7350a02eaa7dd2a93749a4a357a0..2ccff37d604819b28138a3b4805e0e466183687d 100644
--- a/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
+++ b/chrome/browser/ui/cocoa/constrained_web_dialog_delegate_mac.mm
@@ -118,8 +118,8 @@ class ConstrainedWebDialogDelegateViewMac :
   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(); }