Refactor monotonic clock interface and use raw tick values in stopwatch

runtime/vm/os_linux.cc

Index: runtime/vm/os_linux.cc
diff --git a/runtime/vm/os_linux.cc b/runtime/vm/os_linux.cc
index 7924be1cfcd7460c55309f25096ddd8021c0ecc7..244e9b72e10985e9b9871dd30a2e190d7dd08bc6 100644
--- a/runtime/vm/os_linux.cc
+++ b/runtime/vm/os_linux.cc
@@ -396,20 +396,32 @@ int64_t OS::GetCurrentTimeMicros() {
 }
 
 
-int64_t OS::GetCurrentMonotonicMicros() {
+int64_t OS::GetCurrentMonotonicTicks() {
   struct timespec ts;
   if (clock_gettime(CLOCK_MONOTONIC, &ts) != 0) {
     UNREACHABLE();
     return 0;
   }
-  // Convert to microseconds.
+  // Convert to nanoseconds.
   int64_t result = ts.tv_sec;
-  result *= kMicrosecondsPerSecond;
-  result += (ts.tv_nsec / kNanosecondsPerMicrosecond);
+  result *= kNanosecondsPerSecond;
+  result += ts.tv_nsec;
   return result;
 }
 
 
+int64_t OS::GetCurrentMonotonicFrequency() {
+  return kNanosecondsPerSecond;
+}
+
+
+int64_t OS::GetCurrentMonotonicMicros() {
+  int64_t ticks = GetCurrentMonotonicTicks();
+  ASSERT(GetCurrentMonotonicFrequency() == kNanosecondsPerSecond);
+  return ticks / kNanosecondsPerMicrosecond;
+}
+
+
 void* OS::AlignedAllocate(intptr_t size, intptr_t alignment) {
   const int kMinimumAlignment = 16;
   ASSERT(Utils::IsPowerOfTwo(alignment));