Add support for DocumentFragment.getElementById()

Source/core/dom/ContainerNode.cpp

Index: Source/core/dom/ContainerNode.cpp
diff --git a/Source/core/dom/ContainerNode.cpp b/Source/core/dom/ContainerNode.cpp
index f355f618b05a5d2abd7d3807fca55973c52daf0d..f4be545c83d489d59034f896d7663954a12dc620 100644
--- a/Source/core/dom/ContainerNode.cpp
+++ b/Source/core/dom/ContainerNode.cpp
@@ -1162,6 +1162,26 @@ PassRefPtr<RadioNodeList> ContainerNode::radioNodeList(const AtomicString& name,
     return ensureRareData().ensureNodeLists().addCache<RadioNodeList>(*this, type, name);
 }
 
+Element* ContainerNode::getElementById(const AtomicString& id) const
+{
+    if (isInTreeScope()) {
+        // Fast path if we are in a tree scope: call getElementById() on tree scope
+        // and check if the matching element is in our subtree.
+        Element* element = treeScope().getElementById(id);
+        if (!element)
+            return 0;
+        if (element->isDescendantOf(this))

[arv (Not doing code reviews), 2014/04/25 15:28:59]

Can this ever be false?

[Inactive, 2014/04/25 15:31:30]

Yes, just because there is an element in the Document (TreeScope) with the given
ID, doesn't mean it is part of our subtree. As the spec says, we are only
interested in Elements that are in our subtree.

[arv (Not doing code reviews), 2014/04/25 15:37:42]

I see. The SVGSVGElement makes this != tree scope. Good thing isDescendantOf
uses treeScope so we do not have to walk the parent nodes here.

+            return element;
+    }
+
+    // Fall back to traversing our subtree. In case of duplicate ids, the first element found will be returned.
+    for (Element* element = ElementTraversal::firstWithin(*this); element; element = ElementTraversal::next(*element, this)) {
+        if (element->getIdAttribute() == id)
+            return element;
+    }
+    return 0;
+}
+
 #ifndef NDEBUG
 bool childAttachedAllowedWhenAttachingChildren(ContainerNode* node)
 {