Enable system trace clock use on chrome os.

base/time_posix.cc

Index: base/time_posix.cc
diff --git a/base/time_posix.cc b/base/time_posix.cc
index b8f138511486701de9b427e007d2d1ca1b543bdd..1c9645155b1169b56592e1ccd24ee5fb44ae4cc3 100644
--- a/base/time_posix.cc
+++ b/base/time_posix.cc
@@ -24,6 +24,12 @@ namespace base {
 #if defined(OS_ANDROID)
 #define _POSIX_MONOTONIC_CLOCK 1
 #endif
+#if defined(OS_CHROMEOS)
+// Force definition of the system trace clock; it is a chromeos-only api
+// at the moment and surfacing it in the right place requires mucking
+// with glibc et al.
+#define CLOCK_SYSTEM_TRACE 11
+#endif
 
 struct timespec TimeDelta::ToTimeSpec() const {
   int64 microseconds = InMicroseconds();