Make a channel and time limited persistent PreRead experiment

chrome/common/chrome_constants.h

 // Copyright (c) 2011 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.
 
 // A handful of resource-like constants related to the Chrome application.
 
 #ifndef CHROME_COMMON_CHROME_CONSTANTS_H_
 #define CHROME_COMMON_CHROME_CONSTANTS_H_
 #pragma once
 
@@ skipping 98 matching lines @@
 // Are touch icons enabled? False by default.
 extern const bool kEnableTouchIcon;
 
 #if defined(OS_LINUX)
 // The highest and lowest assigned OOM score adjustment
 // (oom_score_adj) used by the OomPriority Manager.
 extern const int kLowestRendererOomScore;
 extern const int kHighestRendererOomScore;
 #endif
 
+#if defined(OS_WIN)
+// This is used by the PreRead experiment.
+extern const char kPreReadEnvironmentVariable[];
+#endif
+
 }  // namespace chrome
 
 #endif  // CHROME_COMMON_CHROME_CONSTANTS_H_