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

webkit/plugins/npapi/webplugin_delegate_impl_mac.mm

Index: webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
diff --git a/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm b/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
index ff23bcbb3a67ca349e8e9188a3d13a08e8ee659d..ba203f97536d6d456527153745d7c10cf8356f1e 100644
--- a/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
+++ b/webkit/plugins/npapi/webplugin_delegate_impl_mac.mm
@@ -460,11 +460,13 @@ void WebPluginDelegateImpl::WindowlessPaint(gfx::NativeDrawingContext 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 = damage_rect;
+    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 = window_rect_;
+    paint_rect.Intersect(damage_rect);
   }
 
   ScopedActiveDelegate active_delegate(this);