DevTools: defer styles delta calculation to until the end of the task.

Source/core/inspector/InspectorController.h

Index: Source/core/inspector/InspectorController.h
diff --git a/Source/core/inspector/InspectorController.h b/Source/core/inspector/InspectorController.h
index 701e88309cd3177e92542eb9af5b4f267f9335cb..b3533a63c8b4f255d70ef8302bc24bc6bc0356f9 100644
--- a/Source/core/inspector/InspectorController.h
+++ b/Source/core/inspector/InspectorController.h
@@ -48,6 +48,7 @@ class InjectedScriptManager;
 class InspectorBackendDispatcher;
 class InspectorAgent;
 class InspectorClient;
+class InspectorCSSAgent;
 class InspectorDOMAgent;
 class InspectorFrontend;
 class InspectorFrontendChannel;
@@ -147,6 +148,7 @@ private:
     OwnPtr<InspectorOverlay> m_overlay;
 
     InspectorDOMAgent* m_domAgent;
+    InspectorCSSAgent* m_cssAgent;
     InspectorPageAgent* m_pageAgent;
     InspectorTimelineAgent* m_timelineAgent;
     InspectorLayerTreeAgent* m_layerTreeAgent;