Add interface to system trace clock.

base/time.h

 // Copyright (c) 2011 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.
 
 // Time represents an absolute point in time, internally represented as
 // microseconds (s/1,000,000) since a platform-dependent epoch.  Each
 // platform's epoch, along with other system-dependent clock interface
 // routines, is defined in time_PLATFORM.cc.
 //
 // TimeDelta represents a duration of time, internally represented in
@@ skipping 464 matching lines @@
   // The resolution of this clock is ~1-15ms.  Resolution varies depending
   // on hardware/operating system configuration.
   static TimeTicks Now();
 
   // Returns a platform-dependent high-resolution tick count. Implementation
   // is hardware dependent and may or may not return sub-millisecond
   // resolution.  THIS CALL IS GENERALLY MUCH MORE EXPENSIVE THAN Now() AND
   // 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();
 
   // Returns true if the high resolution clock is working on this system.
   // This is only for testing.
   static bool IsHighResClockWorking();
 #endif
 
   // Returns true if this object has not been initialized.
@@ skipping 80 matching lines @@
 #endif
 };
 
 inline TimeTicks TimeDelta::operator+(TimeTicks t) const {
   return TimeTicks(t.ticks_ + delta_);
 }
 
 }  // namespace base
 
 #endif  // BASE_TIME_H_