Add timeout expectation for fetch-event.https.html

The test is currently flaky on the Mac trybot.

Bug: 847565
Change-Id: Ic40919118fcd7a2bbc0c0d34a33fac9c3c554aea
Tbr: falken@chromium.org
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1077198Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562586}

third_party/WebKit/LayoutTests/TestExpectations

@@ -1897,6 +1897,8 @@ crbug.com/771118 virtual/service-worker-servicification/external/wpt/service-wor
 
 crbug.com/846237 virtual/service-worker-servicification/external/wpt/service-workers/service-worker/import-scripts-updated-flag.https.html [ Crash ]
 
+crbug.com/847565 [ Mac ] virtual/service-worker-servicification/external/wpt/service-workers/service-worker/fetch-event.https.html [ Pass Timeout ]
+
 crbug.com/492664 [ Linux ] external/wpt/css/css-writing-modes/box-offsets-rel-pos-vlr-005.xht [ Failure ]
 crbug.com/492664 [ Linux ] external/wpt/css/css-writing-modes/box-offsets-rel-pos-vrl-004.xht [ Failure ]
 crbug.com/492664 [ Mac ] external/wpt/css/css-writing-modes/bidi-embed-002.html [ Failure ]