Mark test_resource_timing.html fail on Mac

TBR=skyostil@chromium.org
NOTRY=true

Bug: 799127
Change-Id: Ib5e99e16a5f49eea7e51fa34b9f901c9c143df76
Reviewed-on: https://chromium-review.googlesource.com/850541Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527047}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3555,3 +3555,4 @@ crbug.com/798592 [ Win7 ] virtual/threaded/http/tests/devtools/tracing/timeline-
 
 # Sheriff failures 2018-01-03
 crbug.com/798680 external/wpt/pointerevents/pointerlock/pointerevent_pointerlock_supercedes_capture-manual.html [ Pass Timeout ]
+crbug.com/799127 [ Mac ] external/wpt/resource-timing/test_resource_timing.html [ Failure ]