Make gfx::Rect class operations consistently mutate the class they are called on.

ui/gfx/screen_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 "ui/gfx/screen.h"
 
 #import <ApplicationServices/ApplicationServices.h>
 #import <Cocoa/Cocoa.h>
 
 #include "base/logging.h"
@@ skipping 21 matching lines @@
 }
 
 NSScreen* GetMatchingScreen(const gfx::Rect& match_rect) {
   // Default to the monitor with the current keyboard focus, in case
   // |match_rect| is not on any screen at all.
   NSScreen* max_screen = [NSScreen mainScreen];
   int max_area = 0;
 
   for (NSScreen* screen in [NSScreen screens]) {
     gfx::Rect monitor_area = ConvertCoordinateSystem([screen frame]);
-    gfx::Rect intersection = monitor_area.Intersect(match_rect);
+    gfx::Rect intersection = monitor_area;
+    intersection.Intersect(match_rect);
     int area = intersection.width() * intersection.height();
     if (area > max_area) {
       max_area = area;
       max_screen = screen;
     }
   }
 
   return max_screen;
 }
 
@@ skipping 106 matching lines @@
   NSArray* screens = [NSScreen screens];
   NSScreen* primary = [screens objectAtIndex:0];
   for (NSScreen* screen in screens) {
     if (NSMouseInRect(ns_point, [screen frame], NO))
       return GetDisplayForScreen(screen, screen == primary);
   }
   return GetPrimaryDisplay();
 }
 
 }  // namespace gfx