[MacViews] Use NSTitledWindowMask for window modal dialogs.

ui/views/widget/native_widget_mac.mm

 // Copyright 2014 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 "ui/views/widget/native_widget_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/mac/foundation_util.h"
 #include "base/mac/scoped_nsobject.h"
@@ skipping 20 matching lines @@
 }
 
 + (void)closeWindowWithAnimation:(NSWindow*)window;
 
 @end
 
 namespace views {
 namespace {
 
 NSInteger StyleMaskForParams(const Widget::InitParams& params) {
+  // If the Widget is modal, it will be displayed as a sheet. This works best if
+  // it has NSTitledWindowMask. For example, with NSBorderlessWindowMask, the
+  // parent window still accepts input.
+  if (params.delegate &&
+      params.delegate->GetModalType() == ui::MODAL_TYPE_WINDOW)
+    return NSTitledWindowMask;
+
   // TODO(tapted): Determine better masks when there are use cases for it.
   if (params.remove_standard_frame)
     return NSBorderlessWindowMask;
 
   if (params.type == Widget::InitParams::TYPE_WINDOW) {
     return NSTitledWindowMask | NSClosableWindowMask |
            NSMiniaturizableWindowMask | NSResizableWindowMask |
            NSTexturedBackgroundWindowMask;
   }
   return NSBorderlessWindowMask;
@@ skipping 649 matching lines @@
   [[ViewsNSWindowCloseAnimator alloc] initWithWindow:window];
 }
 
 - (void)animationDidEnd:(NSAnimation*)animation {
   [window_ close];
   [animation_ setDelegate:nil];
   [self release];
 }
 
 @end