Mark breakpoint-with-sourcemap-dart.js as timing out.

This test has been consistently timing out on Mac, so this CL marks that
in the TestExpectations.

https://ci.chromium.org/p/chromium/builders/ci/Mac10.13%20Tests%20%28dbg%29/19844

TBR=gogerald@chromium.org

Bug: 1111287
Change-Id: Ia0aeaaa6f09cae3e27544200a444e5eb6611bbc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380424Reviewed-by: Scott Little <sclittle@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802359}

third_party/blink/web_tests/TestExpectations

@@ -6583,7 +6583,7 @@ crbug.com/1011811 http/tests/devtools/modules-load-initial.js [ Pass Failure ]
 crbug.com/1092794 fast/canvas/OffscreenCanvas-2d-placeholder-willReadFrequently.html [ Pass Failure ]
 
 # Sheriff 2020-07-30
-crbug.com/1111287 http/tests/devtools/sources/debugger-breakpoints/breakpoint-with-sourcemap-dart.js [ Pass Failure ]
+crbug.com/1111287 http/tests/devtools/sources/debugger-breakpoints/breakpoint-with-sourcemap-dart.js [ Pass Failure Timeout ]
 
 crbug.com/1111410 http/tests/serviceworker/service-worker-gc.html [ Pass Failure ]