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

base/time/time_win.cc

Index: base/time/time_win.cc
diff --git a/base/time/time_win.cc b/base/time/time_win.cc
index dc968ad63980edbbe0280519a654447c121a74ae..c52df9c458238cf1bdb596cc827a4690f2953663 100644
--- a/base/time/time_win.cc
+++ b/base/time/time_win.cc
@@ -403,6 +403,8 @@ TimeDelta InitialNowFunction();
 // See "threading notes" in InitializeNowFunctionPointer() for details on how
 // concurrent reads/writes to these globals has been made safe.
 NowFunction g_now_function = &InitialNowFunction;
+TimeTicks::Clock g_now_clock = TimeTicks::WIN_ROLLOVER_PROTECTED_TIME_GET_TIME;
+
 int64_t g_qpc_ticks_per_second = 0;
 
 // As of January 2015, use of <atomic> is forbidden in Chromium code. This is
@@ -459,12 +461,15 @@ void InitializeNowFunctionPointer() {
   // Otherwise, Now uses the high-resolution QPC clock. As of 21 August 2015,
   // ~72% of users fall within this category.
   NowFunction now_function;
+  const char* now_clock;
   base::CPU cpu;
   if (ticks_per_sec.QuadPart <= 0 ||
       !cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
     now_function = &RolloverProtectedNow;
+    now_clock = Clock::WIN_ROLLOVER_PROTECTED_TIME_GET_TIME;
   } else {
     now_function = &QPCNow;
+    now_clock = Clock::WIN_QPC;
   }
 
   // Threading note 1: In an unlikely race condition, it's possible for two or
@@ -476,6 +481,7 @@ void InitializeNowFunctionPointer() {
   // perspective of other threads using the function pointers, that the
   // assignment to |g_qpc_ticks_per_second| happens before the function pointers
   // are changed.
+  g_now_clock = now_clock;
   g_qpc_ticks_per_second = ticks_per_sec.QuadPart;
   ATOMIC_THREAD_FENCE(memory_order_release);
   g_now_function = now_function;
@@ -512,6 +518,13 @@ bool TimeTicks::IsHighResolution() {
 }
 
 // static
+TimeTicks::Clock TimeTicks::GetClock() {

[fmeawad, 2016/03/30 20:25:19]

nit: I might be confused, but why not something like that instead:
return IsHighResolution? WIN_QPC : WIN_ROLLOVER_PROTECTED_TIME_GET_TIME;

[danakj, 2016/03/30 20:26:29]

Oh, that's pretty nice yah.

[charliea (OOO until 10-5), 2016/03/31 15:38:38]

Agreed. That's a much nicer way of doing this. Done.

+  if (g_now_function == &InitialNowFunction)
+    InitializeNowFunctionPointer();
+  return g_now_clock;
+}
+
+// static
 ThreadTicks ThreadTicks::Now() {
   DCHECK(IsSupported());