Add non-member non-mutating methods for common gfx::Rect operations.

ui/gfx/rect.cc

 // 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 "ui/gfx/rect.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
 #elif defined(TOOLKIT_GTK)
 #include <gdk/gdk.h>
@@ skipping 70 matching lines @@
   return r;
 }
 #endif
 
 std::string Rect::ToString() const {
   return base::StringPrintf("%s %s",
                             origin().ToString().c_str(),
                             size().ToString().c_str());
 }
 
+Rect IntersectRects(const Rect& a, const Rect& b) {
+  Rect result = a;
+  result.Intersect(b);
+  return result;
+}
+
+Rect UnionRects(const Rect& a, const Rect& b) {
+  Rect result = a;
+  result.Union(b);
+  return result;
+}
+
+Rect SubtractRects(const Rect& a, const Rect& b) {
+  Rect result = a;
+  result.Subtract(b);
+  return result;
+}
+
 }  // namespace gfx