Reapply 169371 "DevTools: defer styles delta calculation to until the end of the task"

Source/core/inspector/InspectorBaseAgent.h

Index: Source/core/inspector/InspectorBaseAgent.h
diff --git a/Source/core/inspector/InspectorBaseAgent.h b/Source/core/inspector/InspectorBaseAgent.h
index af1c670d086ff633c3a860de421ae45e070cedfc..eae8210979a35633618be2aa14522043f1123730 100644
--- a/Source/core/inspector/InspectorBaseAgent.h
+++ b/Source/core/inspector/InspectorBaseAgent.h
@@ -55,6 +55,7 @@ public:
     virtual void registerInDispatcher(InspectorBackendDispatcher*) = 0;
     virtual void discardAgent() { }
     virtual void didCommitLoadForMainFrame() { }
+    virtual void flushPendingFrontendMessages() { }
 
     String name() { return m_name; }
     void appended(InstrumentingAgents*, InspectorState*);
@@ -77,6 +78,7 @@ public:
     void restore();
     void registerInDispatcher(InspectorBackendDispatcher*);
     void discardAgents();
+    void flushPendingFrontendMessages();
 
 private:
     InstrumentingAgents* m_instrumentingAgents;