[test] Mark test as flaky.

TBR=yangguo@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1125123002

Cr-Commit-Position: refs/heads/master@{#28249}

test/cctest/cctest.status

@@ -132,6 +132,9 @@
   'test-api/Threading3': [PASS, ['mode == debug', SLOW]],
   'test-api/Threading4': [PASS, ['mode == debug', SLOW]],
   'test-strings/StringOOM*': [PASS, ['mode == debug', SKIP]],
+
+  # TODO(machenbach, mvstanton): Flaky in debug on all platforms.
+  'test-lockers/LockerUnlocker': [PASS, ['mode == debug', FLAKY]],
 }],  # ALWAYS
 
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