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..0ce43e078c12216a37c8ec9bee854d1f9e2fb677 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.

[jar (doing other things), 2012/04/27 22:50:18]

What do you mean by "Inside Chrome" vs outside?  Do you mean other processes,
such as renderer processes, etc?

[Sam Leffler, 2012/04/27 22:56:07]

Anything outside chrome the application.  Events coming from the kernel, X
server, etc.

+  static TimeTicks NowFromSystemTraceTime();
+
 #if defined(OS_WIN)
   // Get the absolute value of QPC time drift. For testing.
   static int64 GetQPCDriftMicroseconds();