Make the SystemMonitor observer list thread safe....

base/time_win.cc

 // Copyright (c) 2006-2008 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.
 
 
 // Windows Timer Primer
 //
 // A good article:  http://www.ddj.com/windows/184416651
 // A good mozilla bug:  http://bugzilla.mozilla.org/show_bug.cgi?id=363258
 //
@@ skipping 215 matching lines @@
     : rollover_(TimeDelta::FromMilliseconds(0)),
       last_seen_(0), 
       hi_res_clock_enabled_(false) {
     base::SystemMonitor* system = base::SystemMonitor::Get();
     system->AddObserver(this);
     UseHiResClock(!system->BatteryPower());
   }
 
   ~NowSingleton() {
     UseHiResClock(false);
-    base::SystemMonitor::Get()->RemoveObserver(this);
+    base::SystemMonitor* monitor = base::SystemMonitor::Get();
+    if (monitor)
+      monitor->RemoveObserver(this);
   }
 
   TimeDelta Now() {
     AutoLock locked(lock_);
     // We should hold the lock while calling tick_function to make sure that
     // we keep our last_seen_ stay correctly in sync.
     DWORD now = tick_function();
     if (now < last_seen_)
       rollover_ += TimeDelta::FromMilliseconds(0x100000000I64);  // ~49.7 days.
     last_seen_ = now;
@@ skipping 139 matching lines @@
 
 // static
 TimeTicks TimeTicks::Now() {
   return TimeTicks() + Singleton<NowSingleton>::get()->Now();
 }
 
 // static
 TimeTicks TimeTicks::HighResNow() {
   return TimeTicks() + Singleton<HighResNowSingleton>::get()->Now();
 }