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

chrome/browser/ui/gtk/browser_toolbar_gtk.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 "chrome/browser/ui/gtk/browser_toolbar_gtk.h"
 
 #include <gdk/gdkkeysyms.h>
 #include <gtk/gtk.h>
 #include <X11/XF86keysym.h>
 
@@ skipping 506 matching lines @@
                   allocation.y - kShadowThickness,
                   kCornerWidth,
                   allocation.height + kShadowThickness);
   gfx::Rect left(allocation.x - kShadowThickness,
                  allocation.y - kShadowThickness,
                  kCornerWidth,
                  allocation.height + kShadowThickness);
 
   if (window_->ShouldDrawContentDropShadow()) {
     // Leave room to draw rounded corners.
-    area = area.Subtract(right).Subtract(left);
+    area.Subtract(right);
+    area.Subtract(left);
   }
 
   gfx::Image background = theme_service_->GetImageNamed(IDR_THEME_TOOLBAR);
   background.ToCairo()->SetSource(
       cr, widget, tabstrip_origin.x(), tabstrip_origin.y());
   cairo_pattern_set_extend(cairo_get_source(cr), CAIRO_EXTEND_REPEAT);
   cairo_rectangle(cr, area.x(), area.y(), area.width(), area.height());
   cairo_fill(cr);
 
   if (!window_->ShouldDrawContentDropShadow()) {
@@ skipping 159 matching lines @@
   const gfx::ImageSkia* badge = theme_service_->GetImageSkiaNamed(resource_id);
   gfx::CanvasSkiaPaint canvas(expose, false);
   int x_offset = base::i18n::IsRTL() ? 0 : allocation.width - badge->width();
   int y_offset = 0;
   canvas.DrawImageInt(*badge,
                       allocation.x + x_offset,
                       allocation.y + y_offset);
 
   return FALSE;
 }