Mark custom-elements/form-submission-file.html flaky

TBR=hiroshige@chromium.org

Bug: 910139
No-Try: true
Change-Id: Id37fd777ff192446fd37a1eec58f298b82334ca8
Reviewed-on: https://chromium-review.googlesource.com/c/1356170
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612536}

third_party/blink/web_tests/TestExpectations

@@ -5753,6 +5753,7 @@ crbug.com/908276 [ Mac ] external/wpt/webstorage/storage_setitem.html [ Pass Tim
 crbug.com/908347 [ Mac ] media/autoplay/webaudio-audio-context-resume.html [ Failure Pass ]
 
 # Sheriff 2018-11-29
+crbug.com/910139 custom-elements/form-submission-file.html [ Crash Pass ]
 crbug.com/905772 [ Linux ] inspector-protocol/memory/sampling-native-profile-blink-gc.js [ Failure Pass ]
 crbug.com/910263 [ Mac ] css1/font_properties/font.html [ Pass Crash Failure ]
 crbug.com/910263 [ Mac ] css1/font_properties/font_family.html [ Pass Crash Failure ]