time: Add a static TimeTicks method that returns the underlying clock

base/time/time_posix.cc

Index: base/time/time_posix.cc
diff --git a/base/time/time_posix.cc b/base/time/time_posix.cc
index 4aadee618a0fb3a452edf945ab4b7e9fc3ac8a14..32614bc086d2e015782470f851e76573c07e5065 100644
--- a/base/time/time_posix.cc
+++ b/base/time/time_posix.cc
@@ -312,6 +312,11 @@ TimeTicks TimeTicks::Now() {
 }
 
 // static
+TimeTicks::Clock TimeTicks::GetClock() {
+  return Clock::LINUX_CLOCK_MONOTONIC;
+}
+
+// static
 bool TimeTicks::IsHighResolution() {
   return true;
 }