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

webkit/plugins/ppapi/ppb_scrollbar_impl.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 "webkit/plugins/ppapi/ppb_scrollbar_impl.h"
 
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
 #include "ppapi/c/dev/ppp_scrollbar_dev.h"
@@ skipping 193 matching lines @@
                                 PP_FromBool(IsOverlay()));
 }
 
 void PPB_Scrollbar_Impl::invalidateScrollbarRect(
     WebKit::WebPluginScrollbar* scrollbar,
     const WebKit::WebRect& rect) {
   gfx::Rect gfx_rect(rect.x,
                      rect.y,
                      rect.width,
                      rect.height);
-  dirty_ = dirty_.Union(gfx_rect);
+  dirty_.Union(gfx_rect);
   // Can't call into the client to tell them about the invalidate right away,
   // since the PPB_Scrollbar_Impl code is still in the middle of updating its
   // internal state.
   // Note: we use a WeakPtrFactory here so that a lingering callback can not
   // modify the lifetime of this object. Otherwise, WebKit::WebPluginScrollbar
   // could outlive WebKit::WebPluginContainer, which is against its contract.
   MessageLoop::current()->PostTask(
       FROM_HERE,
       base::Bind(&PPB_Scrollbar_Impl::NotifyInvalidate,
                  weak_ptr_factory_.GetWeakPtr()));
@@ skipping 17 matching lines @@
   pp_rect.point.x = dirty_.x();
   pp_rect.point.y = dirty_.y();
   pp_rect.size.width = dirty_.width();
   pp_rect.size.height = dirty_.height();
   dirty_ = gfx::Rect();
   Invalidate(&pp_rect);
 }
 
 }  // namespace ppapi
 }  // namespace webkit