DevTools: Implement undo, redo operations for the DOMStorage views.

Source/core/inspector/InspectorDOMStorageAgent.cpp

Index: Source/core/inspector/InspectorDOMStorageAgent.cpp
diff --git a/Source/core/inspector/InspectorDOMStorageAgent.cpp b/Source/core/inspector/InspectorDOMStorageAgent.cpp
index fd079bb7778a3204c4afe23cbf0048b354637a2f..675fff62f93b41d6f6fe6b5ce6146d2f608d662c 100644
--- a/Source/core/inspector/InspectorDOMStorageAgent.cpp
+++ b/Source/core/inspector/InspectorDOMStorageAgent.cpp
@@ -107,6 +107,22 @@ void InspectorDOMStorageAgent::disable(ErrorString*)
     m_state->setBoolean(DOMStorageAgentState::domStorageAgentEnabled, false);
 }
 
+void InspectorDOMStorageAgent::getValue(ErrorString* errorString, const RefPtr<JSONObject>& storageId, const String& key, TypeBuilder::OptOutput<WTF::String>* value)
+{
+    Frame* frame;
+    OwnPtr<StorageArea> storageArea = findStorageArea(errorString, storageId, frame);
+    if (!storageArea)
+        return;
+
+    TrackExceptionState es;
+    bool keyPresent = storageArea->contains(key, es, frame);
+    if (hadException(es, errorString) || !keyPresent)
+        return;
+
+    *value = storageArea->getItem(key, es, frame);
+    hadException(es, errorString);
+}
+
 void InspectorDOMStorageAgent::getDOMStorageItems(ErrorString* errorString, const RefPtr<JSONObject>& storageId, RefPtr<TypeBuilder::Array<TypeBuilder::Array<String> > >& items)
 {
     Frame* frame;