Convert WindowOpenDisposition to an enum class.

chrome/browser/ui/cocoa/omnibox/omnibox_popup_view_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/cocoa/omnibox/omnibox_popup_view_mac.h"
 
 #include <cmath>
 
 #include "base/mac/mac_util.h"
 #import "base/mac/sdk_forward_declarations.h"
@@ skipping 144 matching lines @@
 }
 
 void OmniboxPopupViewMac::OnMatrixRowClicked(OmniboxPopupMatrix* matrix,
                                              size_t row) {
   OpenURLForRow(row,
                 ui::WindowOpenDispositionFromNSEvent([NSApp currentEvent]));
 }
 
 void OmniboxPopupViewMac::OnMatrixRowMiddleClicked(OmniboxPopupMatrix* matrix,
                                                    size_t row) {
-  OpenURLForRow(row, NEW_BACKGROUND_TAB);
+  OpenURLForRow(row, WindowOpenDisposition::NEW_BACKGROUND_TAB);
 }
 
 const AutocompleteResult& OmniboxPopupViewMac::GetResult() const {
   return model_->result();
 }
 
 void OmniboxPopupViewMac::CreatePopupIfNeeded() {
   if (!popup_) {
     popup_.reset(
         [[NSWindow alloc] initWithContentRect:ui::kWindowSizeDeterminedLater
@@ skipping 183 matching lines @@
   return NSImageFromImageSkia(
       gfx::CreateVectorIcon(vector_icon_id, kIconSize, icon_color));
 }
 
 void OmniboxPopupViewMac::OpenURLForRow(size_t row,
                                         WindowOpenDisposition disposition) {
   DCHECK_LT(row, GetResult().size());
   omnibox_view_->OpenMatch(GetResult().match_at(row), disposition, GURL(),
                            base::string16(), row);
 }