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 8656ada90f6451c89426239fb4d0e0f07ba49828..96e54c2c877f57c04ea6925ea495021387681038 100644
--- a/Source/core/inspector/InspectorDOMStorageAgent.cpp
+++ b/Source/core/inspector/InspectorDOMStorageAgent.cpp
@@ -106,6 +106,22 @@ void InspectorDOMStorageAgent::disable(ErrorString*)
     m_state->setBoolean(DOMStorageAgentState::domStorageAgentEnabled, false);
 }
 
+void InspectorDOMStorageAgent::getDOMStorageItem(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;
+
+    ExceptionCode ec = 0;
+    bool keyPresent = storageArea->contains(key, ec, frame);
+    if (hadException(ec, errorString) || !keyPresent)
+        return;
+
+    *value = storageArea->getItem(key, ec, frame);
+    hadException(ec, errorString);
+}
+
 void InspectorDOMStorageAgent::getDOMStorageItems(ErrorString* errorString, const RefPtr<JSONObject>& storageId, RefPtr<TypeBuilder::Array<TypeBuilder::Array<String> > >& items)
 {
     Frame* frame;