Mark test as flaky in TestExpectations

http/tests/devtools/service-workers/user-agent-override.js has been flaky:
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=blink_web_tests%20(with%20patch)&tests=http%2Ftests%2Fdevtools%2Fservice-workers%2Fuser-agent-override.js

TBR=shimazu@chromium.org
NOTRY=True 

Bug: 1152532
Change-Id: I7d6c4521f9779f4db53f06c218071d53545a533d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558790Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830651}

third_party/blink/web_tests/TestExpectations

@@ -6030,3 +6030,4 @@ crbug.com/1149734 http/tests/devtools/sources/debugger-ui/continue-to-location-m
 crbug.com/1152496 [ Win ] virtual/oopr-canvas2d/fast/canvas/canvas-composite-shadow.html [ Pass Timeout ]
 crbug.com/1087242 http/tests/inspector-protocol/service-worker/network-extrainfo-main-request.js [ Pass Failure Timeout Crash ]
 crbug.com/1149771 [ Linux ] virtual/android/fullscreen/video-scrolled-iframe.html [ Pass Timeout ]
+crbug.com/1152532 http/tests/devtools/service-workers/user-agent-override.js [ Pass Timeout ]
\ No newline at end of file