Make SystemMonitor not a Singleton and move it out of base

base/time.h

Index: base/time.h
diff --git a/base/time.h b/base/time.h
index 8cd752ab756ba3a4a9d41a5550b90581e4ef64c3..33ab2271be5c252b604a4c2a5d3ae5e7868f160d 100644
--- a/base/time.h
+++ b/base/time.h
@@ -242,12 +242,9 @@ class Time {
   static Time FromFileTime(FILETIME ft);
   FILETIME ToFileTime() const;
 
-  // Monitor system power state and disable high resolution timer when we're
-  // on battery. See time_win.cc for more details.
-  static void StartSystemMonitorObserver();
-
-  // Enable high resolution timer unconditionally. Only for test code.
-  static void EnableHiResClockForTests();
+  // Enable or disable Windows high resolution timer. For more details
+  // see comments in time_win.cc. Returns true on success.
+  static bool UseHighResolutionTimer(bool use);
 #endif
 
   // Converts an exploded structure representing either the local time or UTC