Bug 1296509 - Optimize GetFlattenedTreeParent. r=smaug

We need to call it on some hot paths in stylo, and this allows us to do
quick inline check before delegating to the slow path.

dom/html/HTMLLinkElement.cpp

@@ -17,6 +17,7 @@
 #include "nsGenericHTMLElement.h"
 #include "nsGkAtoms.h"
 #include "nsDOMTokenList.h"
+#include "nsIContentInlines.h"
 #include "nsIDocument.h"
 #include "nsIDOMEvent.h"
 #include "nsIDOMStyleSheet.h"