Editing: Let Selection.collapse with null clears selection.

Source/core/editing/DOMSelection.cpp

Index: Source/core/editing/DOMSelection.cpp
diff --git a/Source/core/editing/DOMSelection.cpp b/Source/core/editing/DOMSelection.cpp
index 6012b131dbfb49621d2f7980ef49dffd783e7777..3d06123b7bb1051bf21c53579f7351469100cadc 100644
--- a/Source/core/editing/DOMSelection.cpp
+++ b/Source/core/editing/DOMSelection.cpp
@@ -197,10 +197,14 @@ int DOMSelection::rangeCount() const
 
 void DOMSelection::collapse(Node* node, int offset, ExceptionState& exceptionState)
 {
-    ASSERT(node);
     if (!m_frame)
         return;
 
+    if (!node) {
+        m_frame->selection().clear();
+        return;
+    }
+
     if (offset < 0) {
         exceptionState.throwDOMException(IndexSizeError, String::number(offset) + " is not a valid offset.");
         return;