[TabManager] Enable tab discarding by default.

chrome/browser/memory/tab_manager_unittest.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 <algorithm>
 #include <map>
 #include <vector>
 
@@ skipping 431 matching lines @@
 
   WebContents* null_contents = tab_manager.DiscardWebContentsAt(1, &tabstrip);
   EXPECT_EQ(new_last_active_time, null_contents->GetLastActiveTime());
 
   tabstrip.CloseAllTabs();
   EXPECT_TRUE(tabstrip.empty());
 }
 
 // Test to see if a tab can only be discarded once. On Windows and Mac, this
 // defaults to true unless overridden through a variation parameter. On other
-// platforms, it's always false
+// platforms, it's always false.
 #if defined(OS_WIN) || defined(OS_MACOSX)
 TEST_F(TabManagerTest, CanOnlyDiscardOnce) {
   TabManager tab_manager;
   const std::string kTrialName = features::kAutomaticTabDiscarding.name;
 
   // Not setting the variation parameter.
   {
     bool discard_once_value = tab_manager.CanOnlyDiscardOnce();
     EXPECT_TRUE(discard_once_value);
   }
@@ skipping 198 matching lines @@
   EXPECT_EQ(0u, task_runner->size());
   EXPECT_EQ(0u, tm.notified_renderers_.size());
 
 
   // Clean up the tabstrip.
   tabstrip.CloseAllTabs();
   ASSERT_TRUE(tabstrip.empty());
 }
 
 }  // namespace memory