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

base/time/time_mac.cc

 // Copyright (c) 2012 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.
 
 #include "base/time/time.h"
 
 #include <CoreFoundation/CFDate.h>
 #include <CoreFoundation/CFTimeZone.h>
 #include <mach/mach.h>
 #include <mach/mach_time.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/sysctl.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <time.h>
 
 #include "base/logging.h"
 #include "base/mac/mach_logging.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/mac/scoped_mach_port.h"
 #include "base/macros.h"
 #include "base/numerics/safe_conversions.h"
 #include "build/build_config.h"
 
 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
   // now - KERN_BOOTTIME to get a time difference that is not impacted by clock
   // changes. KERN_BOOTTIME will be updated by the system whenever the system
   // clock change.
   struct timeval boottime;
   int mib[2] = {CTL_KERN, KERN_BOOTTIME};
   size_t size = sizeof(boottime);
   int kr = sysctl(mib, arraysize(mib), &boottime, &size, NULL, 0);
@@ skipping 192 matching lines @@
 TimeTicks TimeTicks::Now() {
   return TimeTicks(ComputeCurrentTicks());
 }
 
 // static
 bool TimeTicks::IsHighResolution() {
   return true;
 }
 
 // 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());
 }
 
 }  // namespace base