DevTools: CSSAgent::enable should report an error if DOM agent hasn't been enabled

Source/core/inspector/InspectorCSSAgent.cpp

Index: Source/core/inspector/InspectorCSSAgent.cpp
diff --git a/Source/core/inspector/InspectorCSSAgent.cpp b/Source/core/inspector/InspectorCSSAgent.cpp
index 5ebff5b4857847c1f6628bfc2f253b64fe3de747..08a9f1bf77b733a359b595b0237f8fa51b43e9f7 100644
--- a/Source/core/inspector/InspectorCSSAgent.cpp
+++ b/Source/core/inspector/InspectorCSSAgent.cpp
@@ -453,8 +453,12 @@ void InspectorCSSAgent::resetNonPersistentData()
     resetPseudoStates();
 }
 
-void InspectorCSSAgent::enable(ErrorString*, PassRefPtrWillBeRawPtr<EnableCallback> prpCallback)
+void InspectorCSSAgent::enable(ErrorString* errorString, PassRefPtrWillBeRawPtr<EnableCallback> prpCallback)
 {
+    if (!m_domAgent->enabled()) {
+        *errorString = "DOM agent needs to be enabled first.";
+        return;
+    }
     m_state->setBoolean(CSSAgentState::cssAgentEnabled, true);
     if (!m_pageAgent->resourceContentLoader()) {
         wasEnabled();