Mark breakpoint-with-sourcemap-dart as timing out on MSAN

The test has timed out 3 times in a row on the MSAN bot. Updating the
test expectations.

TBR=leese@chromium.org

Bug: 1066647
Change-Id: I58df9c2b5f5f8edd3bf92d75099e449ec917c449
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2131037Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755162}

third_party/blink/web_tests/MSANExpectations

@@ -402,3 +402,6 @@ crbug.com/856601 [ Linux ] external/wpt/badging/idlharness.https.any.worker.html
 
 # Sheriff 2020-02-28
 crbug.com/1047818 [ Linux ] external/wpt/permissions-revoke/idlharness.any.worker.html [ Skip ]
+
+# Sheriff 2020-03-31
+crbug.com/1066647 [ Linux ] http/tests/devtools/sources/debugger-breakpoints/breakpoint-with-sourcemap-dart.js [ Timeout ]