Make time_to_first_suspension's default 1800sec

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 212 matching lines @@
 #endif
   // purge-and-suspend param is used for Purge+Suspend finch experiment
   // in the following way:
   // https://docs.google.com/document/d/1hPHkKtXXBTlsZx9s-9U17XC-ofEIzPo9FYbBEc7PPbk/edit?usp=sharing
   std::string purge_and_suspend_time = variations::GetVariationParamValue(
       "PurgeAndSuspend", "purge-and-suspend-time");
   unsigned time_to_first_suspension_sec;
   if (purge_and_suspend_time.empty() ||
       !base::StringToUint(purge_and_suspend_time,
                           &time_to_first_suspension_sec))
-    time_to_first_suspension_sec = 108000;
+    time_to_first_suspension_sec = 1800;

[Wez, 2017/02/17 07:39:17]

Please define this as a constant e.g. kDefaultTimeToFirstSuspendSeconds and
define it with a comment explaining why we have chosen this value.

[tasak, 2017/02/17 12:36:06]

Changed to use kDefaultTimeToFirstSuspensionSeconds, and add a comment.
However, I think, there is no strong reason why the default value is 30
minutes...

[Wez, 2017/02/17 19:03:14]

There doesn't need to be a _string_ reason, just some rationale at all - e.g.
did we choose the value to purge "soon" after a tab becomes idle, but not so
soon that simply switching tabs will cause jank?

[Wez, 2017/02/17 19:26:32]

s/string/strong ;)

   time_to_first_suspension_ =
       base::TimeDelta::FromSeconds(time_to_first_suspension_sec);
 }
 
 void TabManager::Stop() {
   update_timer_.Stop();
   recent_tab_discard_timer_.Stop();
   memory_pressure_listener_.reset();
 }
 
@@ skipping 786 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