Mark breakpoints-ui-in-multiple-workers.js test as flaky

NOTRY=true

Bug: 954319
Change-Id: Id8eeeb53df0dc8747b9f4f96ae9a6f5891eef8d1
TBR: alph@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576173Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652887}

third_party/blink/web_tests/TestExpectations

@@ -6163,3 +6163,4 @@ crbug.com/954628 [ Win ] external/wpt/payment-request/payment-request-canmakepay
 crbug.com/954994 [ Mac ]  images/jpeg-yuv-image-decoding.html [ Pass Failure ]
 crbug.com/954994 [ Mac ]  virtual/exotic-color-space/images/jpeg-yuv-image-decoding.html [ Pass Failure ]
 crbug.com/954994 [ Mac ]  virtual/gpu-rasterization/images/jpeg-yuv-image-decoding.html [ Pass Failure ]
+crbug.com/954319 [ Linux ] http/tests/devtools/sources/debugger-breakpoints/breakpoints-ui-in-multiple-workers.js [ Pass Timeout ]