Subject .mm files to the header sorting presubmit

ui/gfx/canvas_unittest_mac.mm

 // Copyright 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 "ui/gfx/canvas.h"
 
 #include <cmath>
 
 #import <Cocoa/Cocoa.h>
 
+#include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
-#include "base/strings/sys_string_conversions.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/font.h"
 #include "ui/gfx/font_list.h"
 
 namespace gfx {
 
 namespace {
 
 // Returns the pixel width of the string via calling the native method
 // -sizeWithAttributes.
@@ skipping 56 matching lines @@
   float height = kReallyLargeNumber;
 
   FontList font_list;
   Canvas::SizeStringFloat(
       base::UTF8ToUTF16("Test"), font_list, &width, &height, 0, 0);
 
   EXPECT_GT(width, static_cast<int>(width));
 }
 
 }  // namespace gfx