Make timeline use the same clock source as mojo's tracing infrastructure

runtime/vm/os.h

Index: runtime/vm/os.h
diff --git a/runtime/vm/os.h b/runtime/vm/os.h
index 871b23e2670852257e7caacabad237a0e291770b..5285ec8bc956c88429906d4c789b529c5a23c746 100644
--- a/runtime/vm/os.h
+++ b/runtime/vm/os.h
@@ -47,6 +47,9 @@ class OS {
   // from midnight January 1, 1970 UTC.
   static int64_t GetCurrentTimeMicros();
 
+  // Returns the current time used by the tracing infrastructure.
+  static int64_t GetCurrentTraceMicros();
+
   // Returns a cleared aligned array of type T with n entries.
   // Alignment must be >= 16 and a power of two.
   template<typename T>