Merge to upstream r222492.

Change-Id: I6a0a6e90d217a69531ec3bb5ca0a367f39f4a1da

src/chrono.cpp

@@ -46,6 +46,7 @@ system_clock::from_time_t(time_t t) _NOEXCEPT
     return system_clock::time_point(seconds(t));
 }
 
+#ifndef _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 // steady_clock
 
 const bool steady_clock::is_steady;
@@ -108,10 +109,8 @@ steady_clock::now() _NOEXCEPT
 }
 
 #else  // __APPLE__
-// FIXME: We assume that clock_gettime(CLOCK_MONOTONIC) works on
-// non-apple systems.  Instead, we should check _POSIX_TIMERS and
-// _POSIX_MONOTONIC_CLOCK and fall back to something else if those
-// don't exist.
+// FIXME: if _LIBCPP_HAS_NO_MONOTONIC_CLOCK, then clock_gettime isn't going to
+// work. It may be possible to fall back on something else, depending on the system.
 
 // Warning:  If this is not truly steady, then it is non-conforming.  It is
 //  better for it to not exist and have the rest of libc++ use system_clock
@@ -127,6 +126,8 @@ steady_clock::now() _NOEXCEPT
 }
 #endif  // __APPLE__
 
+#endif // !_LIBCPP_HAS_NO_MONOTONIC_CLOCK
+
 }
 
 _LIBCPP_END_NAMESPACE_STD