Close web contents modal dialogs on content load start

chrome/browser/ui/cocoa/autofill/autofill_dialog_cocoa.mm

 // Copyright (c) 2013 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/cocoa/autofill/autofill_dialog_cocoa.h"
 
 #include "base/mac/bundle_locations.h"
 #include "base/mac/scoped_nsobject.h"
 #include "chrome/browser/ui/autofill/autofill_dialog_controller.h"
 #include "chrome/browser/ui/chrome_style.h"
@@ skipping 36 matching lines @@
   // This should only be called once.
   DCHECK(!sheet_controller_.get());
   sheet_controller_.reset([[AutofillDialogWindowController alloc]
        initWithWebContents:controller_->web_contents()
             autofillDialog:this]);
   base::scoped_nsobject<CustomConstrainedWindowSheet> sheet(
       [[CustomConstrainedWindowSheet alloc]
           initWithCustomWindow:[sheet_controller_ window]]);
   constrained_window_.reset(
       new ConstrainedWindowMac(this, controller_->web_contents(), sheet));
+  constrained_window_->SetPreventCloseOnLoadStart(true);
 }
 
 void AutofillDialogCocoa::Hide() {
   [sheet_controller_ hide];
 }
 
 // Closes the sheet and ends the modal loop. Triggers cleanup sequence.
 void AutofillDialogCocoa::PerformClose() {
   constrained_window_->CloseWebContentsModalDialog();
 }
@@ skipping 263 matching lines @@
   [[signInContainer_ view] setHidden:YES];
   [[mainContainer_ view] setHidden:NO];
   [self performLayout];
 }
 
 - (void)modelChanged {
   [mainContainer_ modelChanged];
 }
 
 @end