Update more core classes to use OVERRIDE / FINAL when needed

Source/core/dom/StringCallback.cpp

Index: Source/core/dom/StringCallback.cpp
diff --git a/Source/core/dom/StringCallback.cpp b/Source/core/dom/StringCallback.cpp
index 1a1a5fc74953dfef4b73f365233dd49003abd891..1b0292092cfc21a2187717638c774b636935a3a7 100644
--- a/Source/core/dom/StringCallback.cpp
+++ b/Source/core/dom/StringCallback.cpp
@@ -39,14 +39,14 @@ namespace WebCore {
 
 namespace {
 
-class DispatchCallbackTask : public ExecutionContextTask {
+class DispatchCallbackTask FINAL : public ExecutionContextTask {
 public:
     static PassOwnPtr<DispatchCallbackTask> create(PassOwnPtr<StringCallback> callback, const String& data)
     {
         return adoptPtr(new DispatchCallbackTask(callback, data));
     }
 
-    virtual void performTask(ExecutionContext*)
+    virtual void performTask(ExecutionContext*) OVERRIDE
     {
         m_callback->handleEvent(m_data);
     }