Remove the ability of webpages to specify strings for the onbeforeunload dialog.

chrome/browser/ui/cocoa/javascript_app_modal_dialog_cocoa.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/cocoa/javascript_app_modal_dialog_cocoa.h"
 
 #import <Cocoa/Cocoa.h>
 #include <stddef.h>
 
 #include "base/i18n/rtl.h"
@@ skipping 65 matching lines @@
   DCHECK(dialog);
   self = [super init];
   if (self)
     nativeDialog_ = dialog;
   return self;
 }
 
 - (NSAlert*)alert {
   if (!alert_) {
     alert_.reset([[NSAlert alloc] init]);
-    // Set a blank icon.
-    NSImage* image =
-        [[[NSImage alloc] initWithSize:NSMakeSize(1, 1)] autorelease];
-    [alert_ setIcon:image];
+    if (!nativeDialog_->dialog()->is_before_unload_dialog()) {
+      // Set a blank icon for dialogs with text provided by the page.
+      // "onbeforeunload" dialogs don't have text provided by the page, so it's
+      // OK to use the app icon.
+      NSImage* image =
+          [[[NSImage alloc] initWithSize:NSMakeSize(1, 1)] autorelease];
+      [alert_ setIcon:image];
+    }
   }
   return alert_;
 }
 
 - (void)addTextFieldWithPrompt:(NSString*)prompt {
   DCHECK(!textField_);
   textField_.reset(
       [[NSTextField alloc] initWithFrame:NSMakeRect(0, 0, 300, 22)]);
   [[textField_ cell] setLineBreakMode:NSLineBreakByTruncatingTail];
   [[self alert] setAccessoryView:textField_];
@@ skipping 364 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(ChromeJavaScriptNativeDialogCocoaFactory);
 };
 
 }  // namespace
 
 void InstallChromeJavaScriptNativeDialogFactory() {
   app_modal::JavaScriptDialogManager::GetInstance()->
       SetNativeDialogFactory(
           make_scoped_ptr(new ChromeJavaScriptNativeDialogCocoaFactory));
 }