Mark flaky test as Timeout

This is another flaky test that may timeout similar to the rest. See
bug for details.

TBR=yangguo@chromium.org

Bug: 1046784
Change-Id: Ic0cb69981a299021eb80405976287b43460c4698
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2028851Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736553}

third_party/blink/web_tests/TestExpectations

@@ -6481,6 +6481,7 @@ crbug.com/1046784 http/tests/devtools/bindings/navigator-frame-attach-detach.js
 crbug.com/1046784 http/tests/devtools/forced-layout-in-microtask.js [ Pass Timeout ]
 crbug.com/1046784 http/tests/devtools/application-panel/resources-panel-iframe-idb.js [ Pass Timeout ]
 crbug.com/1046784 http/tests/devtools/application-panel/resources-panel-on-navigation.js [ Pass Timeout ]
+crbug.com/1046784 http/tests/devtools/a11y-axe-core/sources/scope-pane-a11y-test.js [ Pass Timeout ]
 
 # Sheriff 2020-01-28
 crbug.com/1046201 [ Mac ] fast/forms/calendar-picker/calendar-picker-appearance-zoom125.html [ Pass Failure ]