Partially address a FIXME in steady_clock::now()

  http://reviews.llvm.org/D4045

llvm-svn: 216949
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: c59e585126d36051c0913d432ff224ef97a8583e
diff --git a/include/chrono b/include/chrono
index 2c65eee..9229234 100644
--- a/include/chrono
+++ b/include/chrono
@@ -926,6 +926,7 @@
     static time_point from_time_t(time_t __t) _NOEXCEPT;
 };
 
+#ifndef _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 class _LIBCPP_TYPE_VIS steady_clock
 {
 public:
@@ -939,6 +940,9 @@
 };
 
 typedef steady_clock high_resolution_clock;
+#else
+typedef system_clock high_resolution_clock;
+#endif
 
 } // chrono