Refactor monotonic clock interface and use raw tick values in stopwatch

runtime/vm/os.h

Index: runtime/vm/os.h
diff --git a/runtime/vm/os.h b/runtime/vm/os.h
index a403df2bd04b269beb9ff30d4c1cadebd407cf24..cd570c4bcabe650095a21cc7455d064f5dc8f6d5 100644
--- a/runtime/vm/os.h
+++ b/runtime/vm/os.h
@@ -50,6 +50,12 @@ class OS {
   // Returns the current time used by the tracing infrastructure.
   static int64_t GetCurrentMonotonicMicros();
 
+  // Returns the raw clock value from the monotonic clock.
+  static int64_t GetCurrentMonotonicTicks();
+
+  // Returns the frequency of the monotonic clock.
+  static int64_t GetCurrentMonotonicFrequency();
+
   // Returns a cleared aligned array of type T with n entries.
   // Alignment must be >= 16 and a power of two.
   template<typename T>