fix for high resolution timer on windows.

base/message_loop/message_loop.cc

 // Copyright 2013 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 "base/message_loop/message_loop.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/compiler_specific.h"
@@ skipping 598 matching lines @@
 
   if (run_loop_->quit_when_idle_received_)
     pump_->Quit();
 
   // When we return we will do a kernel wait for more tasks.
 #if defined(OS_WIN)
   // On Windows we activate the high resolution timer so that the wait
   // _if_ triggered by the timer happens with good resolution. If we don't
   // do this the default resolution is 15ms which might not be acceptable
   // for some tasks.
-  in_high_res_mode_ = pending_high_res_tasks_ > 0;
-  Time::ActivateHighResolutionTimer(in_high_res_mode_);
+  bool high_res = pending_high_res_tasks_ > 0;
+  if (high_res != in_high_res_mode_) {
+    in_high_res_mode_ = high_res;
+    Time::ActivateHighResolutionTimer(in_high_res_mode_);
+  }
 #endif
   return false;
 }
 
 void MessageLoop::DeleteSoonInternal(const tracked_objects::Location& from_here,
                                      void(*deleter)(const void*),
                                      const void* object) {
   PostNonNestableTask(from_here, Bind(deleter, object));
 }
 
@@ skipping 76 matching lines @@
       persistent,
       mode,
       controller,
       delegate);
 }
 #endif
 
 #endif  // !defined(OS_NACL)
 
 }  // namespace base