Mark two layout tests flaky

This CL marks
virtual/omt-service-worker-startup/http/tests/serviceworker/update-served-from-cache.html
and its non-virtual tests flaky.

TBR=bashi@chromium.org
NOTRY=true

Bug: 1047208
Change-Id: Ic630ad8f072858653e3f199cef41a4a41730da33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031184
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736867}

third_party/blink/web_tests/TestExpectations

@@ -6508,3 +6508,5 @@ crbug.com/1041846 [ Mac ] virtual/gpu/fast/canvas/canvas-filter-origin-clean.htm
 # Sheriff 2020-01-30
 crbug.com/1047126 [ Win ] external/wpt/IndexedDB/keypath-exceptions.htm [ Failure ]
 crbug.com/1047164 [ Mac ] fast/scroll-snap/snaps-after-keyboard-scrolling.html [ Pass Failure ]
+crbug.com/1047208 virtual/omt-service-worker-startup/http/tests/serviceworker/update-served-from-cache.html [ Pass Failure ]
+crbug.com/1047208 http/tests/serviceworker/update-served-from-cache.html [ Pass Failure ]