[TabManager] Enable tab discarding by default.

chrome/browser/memory/tab_manager.cc

 // 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.
 
 #include "chrome/browser/memory/tab_manager.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <set>
@@ skipping 117 matching lines @@
 
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 // TabManager
 
 TabManager::TabManager()
     : discard_count_(0),
       recent_tab_discard_(false),
       discard_once_(false),
+      minimum_protection_time_(base::TimeDelta::FromMinutes(10)),
       browser_tab_strip_tracker_(this, nullptr, nullptr),
       test_tick_clock_(nullptr),
       under_memory_pressure_(false),
       weak_ptr_factory_(this) {
 #if defined(OS_CHROMEOS)
   delegate_.reset(new TabManagerDelegate(weak_ptr_factory_.GetWeakPtr()));
 #endif
   browser_tab_strip_tracker_.Init(
       BrowserTabStripTracker::InitWith::ALL_BROWERS);
 
   // Set up default callbacks. These may be overridden post-construction as
   // testing seams.
   get_current_pressure_level_ = base::Bind(&GetCurrentPressureLevel);
   notify_renderer_process_ = base::Bind(&NotifyRendererProcess);
 }
 
 TabManager::~TabManager() {
   Stop();
 }
 
 void TabManager::Start() {
 #if defined(OS_WIN) || defined(OS_MACOSX)
-  // If the feature is not enabled, do nothing.
+  // Note that discarding is now enabled by default. This check is kept as a
+  // kill switch.
+  // TODO(georgesak): remote this when deemed not needed anymore.
   if (!base::FeatureList::IsEnabled(features::kAutomaticTabDiscarding))
     return;
 
   // Check the variation parameter to see if a tab is to be protected for an
-  // amount of time after being backgrounded. The value is in seconds.
+  // amount of time after being backgrounded. The value is in seconds. Default
+  // is 10 minutes if the variation is absent.
   std::string minimum_protection_time_string =
       variations::GetVariationParamValue(features::kAutomaticTabDiscarding.name,
                                          "MinimumProtectionTime");
   if (!minimum_protection_time_string.empty()) {
     unsigned int minimum_protection_time_seconds = 0;
     if (base::StringToUint(minimum_protection_time_string,
                            &minimum_protection_time_seconds)) {
       if (minimum_protection_time_seconds > 0)
         minimum_protection_time_ =
             base::TimeDelta::FromSeconds(minimum_protection_time_seconds);
@@ skipping 759 matching lines @@
   // platform.
   std::string allow_multiple_discards = variations::GetVariationParamValue(
       features::kAutomaticTabDiscarding.name, "AllowMultipleDiscards");
   return (allow_multiple_discards != "true");
 #else
   return false;
 #endif
 }
 
 }  // namespace memory