Add chrome://flag to enable the Stale Previews timestamp

chrome/browser/flag_descriptions.h

 // Copyright 2017 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 CHROME_BROWSER_FLAG_DESCRIPTIONS_H_
 #define CHROME_BROWSER_FLAG_DESCRIPTIONS_H_
 
 // Includes needed for macros allowing conditional compilation of some strings.
 #include "build/build_config.h"
 #include "build/buildflag.h"
@@ skipping 176 matching lines @@
 extern const char kEnableDataReductionProxyLitePageName[];
 extern const char kEnableDataReductionProxyLitePageDescription[];
 
 extern const char kDataReductionProxyServerAlternative[];
 extern const char kEnableDataReductionProxyServerExperimentName[];
 extern const char kEnableDataReductionProxyServerExperimentDescription[];
 
 extern const char kEnableDataReductionProxySavingsPromoName[];
 extern const char kEnableDataReductionProxySavingsPromoDescription[];
 
+extern const char kEnableStalePreviewsName[];
+extern const char kEnableStalePreviewsDescription[];
+
 extern const char kEnableEnumeratingAudioDevicesName[];
 extern const char kEnableEnumeratingAudioDevicesDescription[];
 
 extern const char kEnableGenericSensorName[];
 extern const char kEnableGenericSensorDescription[];
 
 extern const char kEnableGroupedHistoryName[];
 extern const char kEnableGroupedHistoryDescription[];
 
 extern const char kEnableHDRName[];
@@ skipping 1312 matching lines @@
 #endif  // defined(TOOLKIT_VIEWS) || defined(OS_ANDROID)
 
 // ============================================================================
 // Don't just add flags to the end, put them in the right section in
 // alphabetical order. See top instructions for more.
 // ============================================================================
 
 }  // namespace flag_descriptions
 
 #endif  // CHROME_BROWSER_FLAG_DESCRIPTIONS_H_