DevTools: require enabling CSS domain before running CSS.* commands.

third_party/WebKit/Source/core/inspector/InspectorCSSAgent.h

Index: third_party/WebKit/Source/core/inspector/InspectorCSSAgent.h
diff --git a/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.h b/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.h
index 6b381b27aedbc56262d059f67ad30c05756e0639..81f8b8b53d8d1aaad1a0eff2faa3d3c3b41f9f9c 100644
--- a/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.h
+++ b/third_party/WebKit/Source/core/inspector/InspectorCSSAgent.h
@@ -280,6 +280,8 @@ class CORE_EXPORT InspectorCSSAgent final
   InspectorStyleSheet* InspectorStyleSheetForRule(CSSStyleRule*);
 
   InspectorStyleSheet* ViaInspectorStyleSheet(Document*);
+
+  protocol::Response AssertEnabled();
   protocol::Response AssertInspectorStyleSheetForId(const String&,
                                                     InspectorStyleSheet*&);
   protocol::Response AssertStyleSheetForId(const String&,
@@ -352,6 +354,7 @@ class CORE_EXPORT InspectorCSSAgent final
   Member<CSSStyleSheet> inspector_user_agent_style_sheet_;
 
   int resource_content_loader_client_id_;
+  bool was_enabled_ = false;
 
   friend class InspectorResourceContentLoaderCallback;
   friend class StyleSheetBinder;