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

Source/core/inspector/InspectorCSSAgent.h

Index: Source/core/inspector/InspectorCSSAgent.h
diff --git a/Source/core/inspector/InspectorCSSAgent.h b/Source/core/inspector/InspectorCSSAgent.h
index 176342981981ccaed51d27a4ae271f51aa18a478..dabeb330ef53377cfe1b3424228950e918f2d57e 100644
--- a/Source/core/inspector/InspectorCSSAgent.h
+++ b/Source/core/inspector/InspectorCSSAgent.h
@@ -117,6 +117,7 @@ public:
     void didMutateRules(CSSStyleSheet*);
     void willMutateStyle();
     void didMutateStyle(CSSStyleDeclaration*, bool);
+    void didProcessTask();
 
     void activeStyleSheetsUpdated(Document*);
     void documentDisposed(Document*);
@@ -161,6 +162,7 @@ private:
 
     void updateActiveStyleSheets(Document*, StyleSheetsUpdateType);
     void setActiveStyleSheets(Document*, const Vector<CSSStyleSheet*>&, StyleSheetsUpdateType);
+    void validateFrontendStylesheets();
 
     void collectPlatformFontsForRenderer(RenderText*, HashCountedSet<String>*);
 
@@ -196,6 +198,7 @@ private:
     HashMap<CSSStyleSheet*, RefPtr<InspectorStyleSheet> > m_cssStyleSheetToInspectorStyleSheet;
     typedef HashMap<Document*, OwnPtr<HashSet<CSSStyleSheet*> > > DocumentStyleSheets;
     DocumentStyleSheets m_documentToCSSStyleSheets;
+    HashSet<Document*> m_invalidatedDocuments;
 
     NodeToInspectorStyleSheet m_nodeToInspectorStyleSheet;
     HashMap<RefPtr<Document>, RefPtr<InspectorStyleSheet> > m_documentToViaInspectorStyleSheet; // "via inspector" stylesheets