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

base/time/time.h

Index: base/time/time.h
diff --git a/base/time/time.h b/base/time/time.h
index 066d910833d2091e60a97870daf0aa7c800dc630..720f9a46efddc26c0b26d4c2d93bc5279f9f4911 100644
--- a/base/time/time.h
+++ b/base/time/time.h
@@ -642,6 +642,16 @@ BASE_EXPORT std::ostream& operator<<(std::ostream& os, Time time);
 // Represents monotonically non-decreasing clock time.
 class BASE_EXPORT TimeTicks : public time_internal::TimeBase<TimeTicks> {
  public:
+  // The underlying clock used to generate new TimeTicks.
+  enum class Clock {
+    UNKNOWN,

[danakj, 2016/03/30 19:51:58]

Not sure that you really need the UNKNOWN here. It just makes you write it in
switches and NOTREACHED() which is unneeded complexity. We never actually return
that. I think you could let the global win one just default init to 0.

LGTM otherwise.

[charliea (OOO until 10-5), 2016/03/30 20:07:44]

Done.

+    LINUX_CLOCK_MONOTONIC,
+    IOS_CF_ABSOLUTE_TIME_MINUS_KERN_BOOTTIME,
+    MAC_MACH_ABSOLUTE_TIME,
+    WIN_QPC,
+    WIN_ROLLOVER_PROTECTED_TIME_GET_TIME
+  };
+
   TimeTicks() : TimeBase(0) {
   }
 
@@ -680,6 +690,11 @@ class BASE_EXPORT TimeTicks : public time_internal::TimeBase<TimeTicks> {
   TimeTicks SnappedToNextTick(TimeTicks tick_phase,
                               TimeDelta tick_interval) const;
 
+  // Returns an enum indicating the underlying clock being used to generate
+  // TimeTicks timestamps. This function should only be used for debugging and
+  // logging purposes.
+  static Clock GetClock();
+
 #if defined(OS_WIN)
  protected:
   typedef DWORD (*TickFunctionType)(void);