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

webkit/tools/test_shell/webwidget_host_mac.mm

Index: webkit/tools/test_shell/webwidget_host_mac.mm
diff --git a/webkit/tools/test_shell/webwidget_host_mac.mm b/webkit/tools/test_shell/webwidget_host_mac.mm
index 60a10f531531ea12813c2dd0e669907ca0143b1d..ae9f644f5627e5e98943e5e5fabce024773ee2f9 100644
--- a/webkit/tools/test_shell/webwidget_host_mac.mm
+++ b/webkit/tools/test_shell/webwidget_host_mac.mm
@@ -190,7 +190,7 @@ void WebWidgetHost::Paint() {
   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_);
@@ -201,7 +201,7 @@ void WebWidgetHost::Paint() {
   // 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();