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

chrome/browser/notifications/balloon_collection_impl.cc

Index: chrome/browser/notifications/balloon_collection_impl.cc
diff --git a/chrome/browser/notifications/balloon_collection_impl.cc b/chrome/browser/notifications/balloon_collection_impl.cc
index f8451491ee7b385e2fdb782ea0ba2bd4218ddd75..4057eef542528dbf493139433cd6321e9df6b7aa 100644
--- a/chrome/browser/notifications/balloon_collection_impl.cc
+++ b/chrome/browser/notifications/balloon_collection_impl.cc
@@ -218,7 +218,7 @@ gfx::Rect BalloonCollectionImpl::GetBalloonsBoundingBox() const {
   for (iter = balloons.begin(); iter != balloons.end(); ++iter) {
     gfx::Rect balloon_box = gfx::Rect((*iter)->GetPosition(),
                                       (*iter)->GetViewSize());
-    bounds = bounds.Union(balloon_box);
+    bounds.Union(balloon_box);
   }
 
   return bounds;