Add Chromium-style TimeDelta, Time and TimeTicks classes, and a new ElapsedTimer class.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index 384ae15c96306a79a6916f871b054e376cfda10e..ef87e9d4a1481af171067ee48465d060aee1f8d6 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -206,10 +206,6 @@ class OS {
   // micro-second resolution.
   static int GetUserTime(uint32_t* secs,  uint32_t* usecs);
 
-  // Get a tick counter normalized to one tick per microsecond.
-  // Used for calculating time intervals.
-  static int64_t Ticks();
-
   // Returns current time as the number of milliseconds since
   // 00:00:00 UTC, January 1, 1970.
   static double TimeCurrentMillis();