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

webkit/tools/test_shell/webwidget_host_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.
 
 #import <Cocoa/Cocoa.h>
 
 #include "webkit/tools/test_shell/webwidget_host.h"
 
 #include "base/logging.h"
 #include "skia/ext/platform_canvas.h"
@@ skipping 172 matching lines @@
   [NSGraphicsContext setCurrentContext:
       [NSGraphicsContext graphicsContextWithGraphicsPort:bitmap_context
                                                  flipped:YES]];
 
   webwidget_->animate(0.0);
 
   // This may result in more invalidation
   webwidget_->layout();
 
   // Scroll the canvas if necessary
-  scroll_rect_ = client_rect.Intersect(scroll_rect_);
+  scroll_rect_.Intersect(client_rect);
   if (!scroll_rect_.IsEmpty()) {
     // add to invalidate rect, since there's no equivalent of ScrollDC.
     paint_rect_ = paint_rect_.Union(scroll_rect_);
   }
   ResetScrollRect();
 
   // Paint the canvas if necessary.  Allow painting to generate extra rects the
   // first time we call it.  This is necessary because some WebCore rendering
   // objects update their layout only when painted.
   for (int i = 0; i < 2; ++i) {
-    paint_rect_ = client_rect.Intersect(paint_rect_);
+    paint_rect_.Intersect(client_rect);
     if (!paint_rect_.IsEmpty()) {
       gfx::Rect rect(paint_rect_);
       paint_rect_ = gfx::Rect();
 
 //      DLOG_IF(WARNING, i == 1) << "painting caused additional invalidations";
       PaintRect(rect);
     }
   }
   DCHECK(paint_rect_.IsEmpty());
 
@@ skipping 62 matching lines @@
 void WebWidgetHost::PaintRect(const gfx::Rect& rect) {
 #ifndef NDEBUG
   DCHECK(!painting_);
 #endif
   DCHECK(canvas_.get());
 
   set_painting(true);
   webwidget_->paint(webkit_glue::ToWebCanvas(canvas_.get()), rect);
   set_painting(false);
 }