[Sheriff] Mark snap-to-area-with-fractional-offset.html as flaky on Mac 10.13

TBR=alsan@google.com

Bug: 1036054
Change-Id: Ie24ccf69dcece090f2a523d2ac85a80a7d8ba7e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1980754Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727228}

third_party/blink/web_tests/TestExpectations

@@ -6143,3 +6143,4 @@ crbug.com/859169 http/tests/devtools/layers/layer-compositing-reasons.js [ Pass
 # Sheriff 2019-12-23
 crbug.com/1036626 http/tests/devtools/tracing/tracing-record-input-events.js [ Pass Failure ]
 crbug.com/1036626 virtual/threaded/http/tests/devtools/tracing/tracing-record-input-events.js [ Pass Failure ]
+crbug.com/1036054 [ Mac10.13 ] virtual/threaded/fast/scroll-snap/snap-to-area-with-fractional-offset.html [ Pass Failure ]