Add interface to system trace clock.

base/time.h

Index: base/time.h
diff --git a/base/time.h b/base/time.h
index 8590e993555380faec730a17dfd7f9d93fb3450a..bd50ba4e6e03eb8a03fdfb97cf110af2a7ed60bf 100644
--- a/base/time.h
+++ b/base/time.h
@@ -482,6 +482,13 @@ class BASE_EXPORT TimeTicks {
   // SHOULD ONLY BE USED WHEN IT IS REALLY NEEDED.
   static TimeTicks HighResNow();
 
+  // Returns the current system trace time or, if none is defined, the current
+  // high-res time (i.e. HighResNow()). On systems where a global trace clock
+  // is defined, timestamping TraceEvents's with this value guarantees
+  // synchronization between events collected inside chrome and events
+  // collected outside (e.g. kernel, X server).
+  static TimeTicks NowFromSystemTraceTime();
+
 #if defined(OS_WIN)
   // Get the absolute value of QPC time drift. For testing.
   static int64 GetQPCDriftMicroseconds();