Bug 1466991 - Part 1: Factor out ShouldUseXBLScope.

dom/base/nsIDocument.h

@@ -3439,13 +3439,29 @@ nsINode::OwnerDocAsNode() const
   return OwnerDoc();
 }
 
+// ShouldUseXBLScope is defined here as a template so that we can get the faster
+// version of IsInAnonymousSubtree if we're statically known to be an
+// nsIContent.  we could try defining ShouldUseXBLScope separately on nsINode
+// and nsIContent, but then we couldn't put its nsINode implementation here
+// (because this header does not include nsIContent) and we can't put it in
+// nsIContent.h, because the definition of nsIContent::IsInAnonymousSubtree is
+// in nsIContentInlines.h.  And then we get include hell from people trying to
+// call nsINode::GetParentObject but not including nsIContentInlines.h and with
+// no really good way to include it.
+template<typename T>
+inline bool ShouldUseXBLScope(const T* aNode)
+{
+  return aNode->IsInAnonymousSubtree() &&
+         !aNode->IsAnonymousContentInSVGUseSubtree();
+}
+
 inline mozilla::dom::ParentObject
 nsINode::GetParentObject() const
 {
   mozilla::dom::ParentObject p(OwnerDoc());
     // Note that mUseXBLScope is a no-op for chrome, and other places where we
     // don't use XBL scopes.
-  p.mUseXBLScope = IsInAnonymousSubtree() && !IsAnonymousContentInSVGUseSubtree();
+  p.mUseXBLScope = ShouldUseXBLScope(this);
   return p;
 }