Add interface to system trace clock.

base/time_unittest.cc

Index: base/time_unittest.cc
diff --git a/base/time_unittest.cc b/base/time_unittest.cc
index 9823147a33dac223d224726b3c988676cf8e7be6..b4b166be75ee1c9deaeab9bcb07b09f4ab7e1eaa 100644
--- a/base/time_unittest.cc
+++ b/base/time_unittest.cc
@@ -464,7 +464,7 @@ TEST(TimeTicks, Deltas) {
   }
 }
 
-TEST(TimeTicks, HighResNow) {
+static void HighResClockTest(TimeTicks (*GetTicks)()) {
 #if defined(OS_WIN)
   // HighResNow doesn't work on some systems.  Since the product still works
   // even if it doesn't work, it makes this entire test questionable.
@@ -486,12 +486,12 @@ TEST(TimeTicks, HighResNow) {
   int retries = 100;  // Arbitrary.
   TimeDelta delta;
   while (!success && retries--) {
-    TimeTicks ticks_start = TimeTicks::HighResNow();
+    TimeTicks ticks_start = GetTicks();
     // Loop until we can detect that the clock has changed.  Non-HighRes timers
     // will increment in chunks, e.g. 15ms.  By spinning until we see a clock
     // change, we detect the minimum time between measurements.
     do {
-      delta = TimeTicks::HighResNow() - ticks_start;
+      delta = GetTicks() - ticks_start;
     } while (delta.InMilliseconds() == 0);
 
     if (delta.InMicroseconds() <= kTargetGranularityUs)
@@ -503,6 +503,15 @@ TEST(TimeTicks, HighResNow) {
   EXPECT_TRUE(success);
 }
 
+TEST(TimeTicks, HighResNow) {
+  HighResClockTest(&TimeTicks::HighResNow);
+}
+
+TEST(TimeTicks, NowFromSystemTraceTime) {
+  // Re-use HighResNow test for now since clock properties are identical.
+  HighResClockTest(&TimeTicks::NowFromSystemTraceTime);
+}
+
 TEST(TimeDelta, FromAndIn) {
   EXPECT_TRUE(TimeDelta::FromDays(2) == TimeDelta::FromHours(48));
   EXPECT_TRUE(TimeDelta::FromHours(3) == TimeDelta::FromMinutes(180));