Mark test as flaky in TestExpectations

The test http/tests/inspector-protocol/service-worker/network-extrainfo-main-request.js has been flaky for a while:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests%20(with%20patch)&tests=http/tests/inspector-protocol/service-worker/network-extrainfo-main-request.js

TBR=shimazu@chromium.org
NOTRY=True 

Bug: 1087242
Change-Id: I6e2ee8020f2e67f0ca4f271929bad68776d5672f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558160Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830577}

third_party/blink/web_tests/TestExpectations

@@ -6030,3 +6030,6 @@ crbug.com/1152088 [ Mac10.13 Debug ] fast/dom/cssTarget-crash.html [ Pass Timeou
 crbug.com/1149734 http/tests/devtools/sources/source-frame-toolbar-items.js [ Pass Failure ]
 crbug.com/1149734 http/tests/devtools/sources/debugger-frameworks/frameworks-sourcemap.js [ Pass Failure ]
 crbug.com/1149734 http/tests/devtools/sources/debugger-ui/continue-to-location-markers.js [ Pass Failure ]
+
+#Sheriff 2020-11-24
+crbug.com/1087242 http/tests/inspector-protocol/service-worker/network-extrainfo-main-request.js [ Pass Failure Timeout Crash ]
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