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

base/time/time_mac.cc

Index: base/time/time_mac.cc
diff --git a/base/time/time_mac.cc b/base/time/time_mac.cc
index f0c780449b7b23ae89fbd1cad7d86b3905f88a18..5d7a703d67db11199b01a0f92a9c382e44dbd1aa 100644
--- a/base/time/time_mac.cc
+++ b/base/time/time_mac.cc
@@ -25,6 +25,13 @@
 
 namespace {
 
+#if defined(OS_IOS)
+const char* kIOSKernBoottimeClockId =
+    "IOS_CF_ABSOLUTE_TIME_MINUS_KERN_BOOTTIME_MICROS";
+#else
+const char* kMacMachClockId = "MAC_MACH_ABSOLUTE_TIME_MICROS";
+#endif  // defined(OS_IOS)
+
 int64_t ComputeCurrentTicks() {
 #if defined(OS_IOS)
   // On iOS mach_absolute_time stops while the device is sleeping. Instead use
@@ -237,6 +244,15 @@ bool TimeTicks::IsHighResolution() {
 }
 
 // static
+std::string TimeTicks::ClockId() {
+#if defined(OS_IOS)
+  return kIOSKernBoottimeClockId;
+#else
+  return kMacMachClockId;
+#endif  // defined(OS_IOS)
+}
+
+// static
 ThreadTicks ThreadTicks::Now() {
   return ThreadTicks(ComputeThreadTicks());
 }