BenchSysTimer_mach drops all sub-ms precision.  Fix that!

BUG=skia:
R=bungeman@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/336903005
diff --git a/bench/BenchSysTimer_mach.cpp b/bench/BenchSysTimer_mach.cpp
index cf3f1c1..1f0a6b9 100644
--- a/bench/BenchSysTimer_mach.cpp
+++ b/bench/BenchSysTimer_mach.cpp
@@ -71,6 +71,6 @@
     } else {
         uint64_t elapsedNano = elapsed * sTimebaseInfo.numer
                                / sTimebaseInfo.denom;
-        return elapsedNano / 1000000;
+        return elapsedNano / 1000000.0;
     }
 }