Enable ancient assert in LayoutTreeBuilder.

Source/core/dom/LayoutTreeBuilder.h

Index: Source/core/dom/LayoutTreeBuilder.h
diff --git a/Source/core/dom/LayoutTreeBuilder.h b/Source/core/dom/LayoutTreeBuilder.h
index 6204e801d43f2c8085863c4aeb749cb9a62ab8e1..4a4a11f700812e2a61bb102df1435713f25527a8 100644
--- a/Source/core/dom/LayoutTreeBuilder.h
+++ b/Source/core/dom/LayoutTreeBuilder.h
@@ -48,12 +48,7 @@ protected:
         ASSERT(!node.layoutObject());
         ASSERT(node.needsAttach());
         ASSERT(node.document().inStyleRecalc());
-
-        // FIXME: We should be able to ASSERT(node->inActiveDocument()) but childrenChanged is called
-        // before ChildNodeInsertionNotifier in ContainerNode's methods and some implementations
-        // will trigger a layout inside childrenChanged.
-        // Mainly HTMLTextAreaElement::childrenChanged calls HTMLTextFormControlElement::setSelectionRange
-        // which does an updateLayoutIgnorePendingStylesheets.
+        ASSERT(node.inActiveDocument());
     }
 
     LayoutObject* nextLayoutObject() const