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

content/browser/renderer_host/accelerated_surface_container_mac.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 "content/browser/renderer_host/accelerated_surface_container_mac.h"
 
 #include "base/logging.h"
 #include "content/browser/renderer_host/accelerated_surface_container_manager_mac.h"
 #include "ui/surface/io_surface_support_mac.h"
 #include "webkit/plugins/npapi/webplugin.h"
@@ skipping 210 matching lines @@
 
 void AcceleratedSurfaceContainerMac::set_was_painted_to(uint64 surface_handle) {
   set_was_painted_to_common(surface_handle);
   update_rect_ = gfx::Rect();
 }
 
 void AcceleratedSurfaceContainerMac::set_was_painted_to(
    uint64 surface_handle,
    const gfx::Rect& update_rect) {
   set_was_painted_to_common(surface_handle);
-  update_rect_ = update_rect_.Union(update_rect);
+  update_rect_.Union(update_rect);
 }
 
 void AcceleratedSurfaceContainerMac::EnqueueTextureForDeletion() {
   if (texture_) {
     DCHECK(texture_pending_deletion_ == 0);
     texture_pending_deletion_ = texture_;
     texture_ = 0;
   }
 }
 
@@ skipping 11 matching lines @@
         surface_.reset(surface);
         surface_handle_ = surface_handle;
         surface_width_ = io_surface_support->IOSurfaceGetWidth(surface_);
         surface_height_ = io_surface_support->IOSurfaceGetHeight(surface_);
         EnqueueTextureForDeletion();
       }
     }
   }
   was_painted_to_ = true;
 }