Enable tab discarding on Mac (behind a flag).

chrome/common/chrome_switches.h

 // Copyright (c) 2012 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.
 
 // Defines all the command-line switches used by Chrome.
 
 #ifndef CHROME_COMMON_CHROME_SWITCHES_H_
 #define CHROME_COMMON_CHROME_SWITCHES_H_
 
 #include "build/build_config.h"
@@ skipping 353 matching lines @@
 extern const char kEnableAppWindowCycling[];
 extern const char kEnableMacViewsNativeAppWindows[];
 extern const char kEnableTranslateNewUX[];
 extern const char kMetricsClientID[];
 extern const char kRelauncherProcess[];
 #endif  // defined(OS_MACOSX)
 
 #if defined(OS_WIN)
 extern const char kEnableCloudPrintXps[];
 extern const char kEnableProfileShortcutManager[];
-extern const char kEnableTabDiscarding[];
 extern const char kForceDesktop[];
 extern const char kForceImmersive[];
 extern const char kHideIcons[];
 extern const char kNoNetworkProfileWarning[];
 extern const char kRelaunchShortcut[];
 extern const char kShowIcons[];
 extern const char kUninstall[];
 extern const char kViewerLaunchViaAppId[];
 extern const char kWaitForMutex[];
 extern const char kWindows8Search[];
 #endif  // defined(OS_WIN)
 
+#if defined(OS_WIN) || defined(OS_MACOSX)
+extern const char kEnableTabDiscarding[];
+#endif  // defined(OS_WIN) || defined(OS_MACOSX)
+
 #if defined(ENABLE_IPC_FUZZER)
 extern const char kIpcFuzzerTestcase[];
 #endif
 
 #if defined(ENABLE_PRINT_PREVIEW) && !defined(OFFICIAL_BUILD)
 extern const char kDebugPrint[];
 #endif
 
 #if defined(ENABLE_PLUGINS)
 extern const char kAllowNaClCrxFsAPI[];
@@ skipping 21 matching lines @@
 #if defined(ENABLE_TASK_MANAGER)
 bool NewTaskManagerEnabled();
 #endif  // defined(ENABLE_TASK_MANAGER)
 
 // DON'T ADD RANDOM STUFF HERE. Put it in the main section above in
 // alphabetical order, or in one of the ifdefs (also in order in each section).
 
 }  // namespace switches
 
 #endif  // CHROME_COMMON_CHROME_SWITCHES_H_