Sheriff: disabling flaky/failing test

http/tests/devtools/a11y-axe-core/elements/main-tool-test.js

failing since:

https://ci.chromium.org/p/chromium/builders/ci/WebKit%20Linux%20Trusty%20MSAN/14679

TBR=masonfreed@chromium.org
NOTRY=true

Bug: 966932
Change-Id: Id7001fdb2ff59498256c6297f696f33c9d456692
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628029Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663152}

third_party/blink/web_tests/TestExpectations

@@ -5717,3 +5717,6 @@ crbug.com/965389 [ Mac ] media/track/track-cue-rendering-position-auto.html [ Pa
 
 # Sheriff 2019-05-21
 crbug.com/965134 [ Linux ] virtual/mouseevent_fractional/fast/events/pointerevents/multi-touch-events.html [ Failure ]
+
+# Sheriff 2019-05-24
+crbug.com/966932 [ Linux ] http/tests/devtools/a11y-axe-core/elements/main-tool-test.js [ Timeout Pass ]