Disable additional flaky worker web tests.

This CL disables:
virtual/omt-worker-fetch/external/wpt/workers/modules/dedicated-worker-import.any.worker.html
http/tests/workers/worker-usecounter.html
virtual/omt-worker-fetch/http/tests/workers/worker-usecounter.html

TBR=nhiroki@chromium.org

Bug: 867532
Change-Id: I65e133aef1c9841bfa8b43dd5c10a9b4f866a69d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264054Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781858}

third_party/blink/web_tests/TestExpectations

@@ -6862,3 +6862,6 @@ crbug.com/1098371 [ Linux ] virtual/threaded-no-composited-antialiasing/animatio
 
 # Sheriff 2020-06-24
 crbug.com/1098676 [ Win ] http/tests/devtools/console-xhr-logging.js [ Pass Failure ]
+crbug.com/867532 [ Linux ] virtual/omt-worker-fetch/external/wpt/workers/modules/dedicated-worker-import.any.worker.html [ Pass Timeout ]
+crbug.com/867532 [ Linux ] http/tests/workers/worker-usecounter.html [ Pass Timeout ]
+crbug.com/867532 [ Linux ] virtual/omt-worker-fetch/http/tests/workers/worker-usecounter.html [ Pass Timeout ]