Mark http/tests/devtools/service-workers/service-workers-force-update-on-page-load.js as flaky

Dashboard:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=http%2Ftests%2Fdevtools%2Fservice-workers%2Fservice-workers-force-update-on-page-load.js

TBR=hhli@chromium.org
NOTRY=true

Bug: 937378
Change-Id: I0ec581ee4cad72171592394f1f72052758b9b4bd
Reviewed-on: https://chromium-review.googlesource.com/c/1497174Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636893}

third_party/blink/web_tests/TestExpectations

@@ -5962,3 +5962,4 @@ crbug.com/937170 [ Linux Win7 ] external/wpt/IndexedDB/interleaved-cursors-large
 crbug.com/937312 [ Win7 ] external/wpt/background-fetch/fetch.https.window.html [ Pass Timeout ]
 crbug.com/893659 [ Mac ] virtual/threaded/fast/events/pointerevents/pinch/pointerevent_touch-action-pinch_zoom_touch.html [ Pass Failure Crash ]
 crbug.com/937358 [ Win ] virtual/scroll_customization/fast/events/touch/touch-handler-iframe-plugin-assert.html [ Pass Failure ]
+crbug.com/937378 [ Win7 ] http/tests/devtools/service-workers/service-workers-force-update-on-page-load.js [ Pass Timeout ]