Bug 450340 trivial follow-up: flip constructor's initialization order to fix a build warning. r=roc

layout/base/nsLayoutUtils.cpp

@@ -1390,7 +1390,7 @@ struct BoxToBorderRect : public nsLayoutUtils::BoxCallback {
   nsLayoutUtils::RectCallback* mCallback;
 
   BoxToBorderRect(nsIFrame* aRelativeTo, nsLayoutUtils::RectCallback* aCallback)
-    : mCallback(aCallback), mRelativeTo(aRelativeTo) {}
+    : mRelativeTo(aRelativeTo), mCallback(aCallback) {}
 
   virtual void AddBox(nsIFrame* aFrame) {
 #ifdef MOZ_SVG