selection: Align collapse-NN.html and extend-NN.html to the latest draft.

third_party/WebKit/Source/core/editing/DOMSelection.cpp

Index: third_party/WebKit/Source/core/editing/DOMSelection.cpp
diff --git a/third_party/WebKit/Source/core/editing/DOMSelection.cpp b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
index f14363f72d40bd6c089440af596e4a44d02fcece..a1abcd196b9f0c79a394804ef596e40a4255b0e1 100644
--- a/third_party/WebKit/Source/core/editing/DOMSelection.cpp
+++ b/third_party/WebKit/Source/core/editing/DOMSelection.cpp
@@ -428,11 +428,15 @@ void DOMSelection::modify(const String& alterString,
   frame()->selection().modify(alter, direction, granularity);
 }
 
+// https://www.w3.org/TR/selection-api/#dom-selection-extend
 void DOMSelection::extend(Node* node,
                           int offset,
                           ExceptionState& exceptionState) {
   DCHECK(node);
-  // https://www.w3.org/TR/selection-api/#dom-selection-extend
+  // 1. If node's root is not the document associated with the context object,
+  // abort these steps.
+  if (!isValidForPosition(node))
+    return;
 
   // 2. If the context object is empty, throw an InvalidStateError exception and
   // abort these steps.
@@ -451,11 +455,6 @@ void DOMSelection::extend(Node* node,
   if (exceptionState.hadException())
     return;
 
-  // 1. If node's root is not the document associated with the context object,
-  // abort these steps.
-  if (!isValidForPosition(node))
-    return;
-
   // 3. Let oldAnchor and oldFocus be the context object's anchor and focus, and
   // let newFocus be the boundary point (node, offset).
   const Position& oldAnchor = anchorPosition();