Add Retina to the list of popup-blocking-timers5 failures

The test is also failing on Retina builds:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/WebKit%20Mac10.13%20(retina)/8631

Bug: 917970
Change-Id: I3de376948ec19cfc4a83c58dd28b5bde04fd3f95
Reviewed-on: https://chromium-review.googlesource.com/c/1394868
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619746}

third_party/blink/web_tests/TestExpectations

@@ -6034,7 +6034,7 @@ crbug.com/917284 virtual/disabled-service-worker-servicification/external/wpt/se
 crbug.com/917284 virtual/outofblink-cors/external/wpt/service-workers/service-worker/claim-fetch-with-appcache.https.html [ Failure ]
 
 # Sheriff 2018-12-27
-crbug.com/917970 [ Mac10.13 ] virtual/mouseevent_fractional/fast/events/popup-blocking-timers5.html [ Pass Failure ]
+crbug.com/917970 [ Mac10.13 Retina ] virtual/mouseevent_fractional/fast/events/popup-blocking-timers5.html [ Pass Failure ]
 
 # Sheriff 2019-01-03
 crbug.com/918905 [ Mac10.13 ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/sizing/block-size-with-min-or-max-content-table-1b.html [ Pass Failure ]