Add WebElement::isEditable.

Source/core/dom/Node.cpp

Index: Source/core/dom/Node.cpp
diff --git a/Source/core/dom/Node.cpp b/Source/core/dom/Node.cpp
index 1a14679ed6db74c334cbea4e649086733500dc87..173b9e66cecd5e74e520b3cb5be8aa2449890ba4 100644
--- a/Source/core/dom/Node.cpp
+++ b/Source/core/dom/Node.cpp
@@ -519,13 +519,13 @@ void Node::normalize()
     }
 }
 
-bool Node::isContentEditable(UserSelectAllTreatment treatment)
+bool Node::isContentEditable(UserSelectAllTreatment treatment) const
 {
     document().updateLayoutTreeIfNeeded();
     return hasEditableStyle(Editable, treatment);
 }
 
-bool Node::isContentRichlyEditable()
+bool Node::isContentRichlyEditable() const
 {
     document().updateLayoutTreeIfNeeded();
     return hasEditableStyle(RichlyEditable, UserSelectAllIsAlwaysNonEditable);