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

webkit/plugins/npapi/webplugin_delegate_impl_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 "webkit/plugins/npapi/webplugin_delegate_impl.h"
 
 #import <Cocoa/Cocoa.h>
 #import <QuartzCore/QuartzCore.h>
 #include <unistd.h>
 
@@ skipping 437 matching lines @@
                                             const gfx::Rect& damage_rect) {
   // If we get a paint event before we are completely set up (e.g., a nested
   // call while the plugin is still in NPP_SetWindow), bail.
   if (!have_called_set_window_ || (use_buffer_context_ && !buffer_context_))
     return;
   DCHECK(!use_buffer_context_ || buffer_context_ == context);
 
   base::StatsRate plugin_paint("Plugin.Paint");
   base::StatsScope<base::StatsRate> scope(plugin_paint);
 
-  gfx::Rect paint_rect;
+  gfx::Rect paint_rect = damage_rect;
   if (use_buffer_context_) {
     // Plugin invalidates trigger asynchronous paints with the original
     // invalidation rect; the plugin may be resized before the paint is handled,
     // so we need to ensure that the damage rect is still sane.
-    paint_rect = damage_rect.Intersect(
+    paint_rect.Intersect(
         gfx::Rect(0, 0, window_rect_.width(), window_rect_.height()));
   } else {
     // Use the actual window region when drawing directly to the window context.
-    paint_rect = damage_rect.Intersect(window_rect_);
+    paint_rect.Intersect(window_rect_);
   }
 
   ScopedActiveDelegate active_delegate(this);
 
   CGContextSaveGState(context);
 
   if (!use_buffer_context_) {
     // Reposition the context origin so that plugins will draw at the correct
     // location in the window.
     CGContextClipToRect(context, paint_rect.ToCGRect());
@@ skipping 285 matching lines @@
   DCHECK(!composited_);
   windowed_handle_ = handle;
   surface_->SetWindowHandle(handle);
   UpdateAcceleratedSurface();
   // Kick off the drawing timer, if necessary.
   PluginVisibilityChanged();
 }
 
 }  // namespace npapi
 }  // namespace webkit