Add a FOUC painting test.

Source/web/tests/sim/SimNetwork.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SimNetwork_h
 #define SimNetwork_h
 
 #include "public/platform/WebURLLoaderTestDelegate.h"
 #include "wtf/HashMap.h"
 #include "wtf/text/StringHash.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
 class SimRequest;
 class WebURLLoader;
 class WebURLLoaderClient;
 class WebURLResponse;
 
+// Simulates a network with precise flow control so you can make requests
+// return, write data, and finish in a specific order in a unit test. One of
+// these must be created before using the SimRequest to issue requests.
 class SimNetwork final : public WebURLLoaderTestDelegate {
 public:
     SimNetwork();
     ~SimNetwork();
 
 private:
     friend class SimRequest;
 
     static SimNetwork& current();
 
     void servePendingRequests();
     void addRequest(SimRequest&);
     void removeRequest(SimRequest&);
 
     // WebURLLoaderTestDelegate
     void didReceiveResponse(WebURLLoaderClient*, WebURLLoader*, const WebURLResponse&) override;
     void didReceiveData(WebURLLoaderClient*, WebURLLoader*, const char* data, int dataLength, int encodedDataLength) override;
     void didFail(WebURLLoaderClient*, WebURLLoader*, const WebURLError&) override;
     void didFinishLoading(WebURLLoaderClient*, WebURLLoader*, double finishTime, int64_t totalEncodedDataLength) override;
 
     SimRequest* m_currentRequest;
     HashMap<String, SimRequest*> m_requests;
 };
 
 } // namespace blink
 
 #endif