[TabManager] Enable tab discarding by default.

chrome/common/chrome_features.cc

 // Copyright 2016 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.
 
 #include "chrome/common/chrome_features.h"
 
 namespace features {
 
 // All features in alphabetical order.
 
 #if defined(OS_CHROMEOS)
 // Whether to handle low memory kill of ARC apps by Chrome.
 const base::Feature kArcMemoryManagement{
     "ArcMemoryManagement", base::FEATURE_ENABLED_BY_DEFAULT};
 #endif  // defined(OS_CHROMEOS)
 
 #if defined(OS_WIN) || defined(OS_MACOSX)
 // Enables automatic tab discarding, when the system is in low memory state.
 const base::Feature kAutomaticTabDiscarding{"AutomaticTabDiscarding",
-                                            base::FEATURE_DISABLED_BY_DEFAULT};
+                                            base::FEATURE_ENABLED_BY_DEFAULT};
 #endif  // defined(OS_WIN) || defined(OS_MACOSX)
 
 #if defined(OS_WIN) || defined(OS_LINUX)
 // Enables the Restart background mode optimization. When all Chrome UI is
 // closed and it goes in the background, allows to restart the browser to
 // discard memory.
 const base::Feature kBackgroundModeAllowRestart{
     "BackgroundModeAllowRestart", base::FEATURE_DISABLED_BY_DEFAULT};
 #endif  // defined(OS_WIN) || defined(OS_LINUX)
 
@@ skipping 113 matching lines @@
 // Enables or disables PIN quick unlock settings integration.
 const base::Feature kQuickUnlockPin{"QuickUnlockPin",
                                     base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enables or disables PIN quick unlock settings integration.
 const base::Feature kEHVInputOnImeMenu{"EmojiHandwritingVoiceInput",
                                        base::FEATURE_DISABLED_BY_DEFAULT};
 #endif  // defined(OS_CHROMEOS)
 
 }  // namespace features