Mark ProcessDebugMessagesThreaded as flaky.

test/cctest/cctest.status

 # Copyright 2011 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 69 matching lines @@
   'test-cpu-profiler/BoundFunctionCall': [SKIP],
   'test-cpu-profiler/CpuProfileDeepStack': [SKIP],
   'test-cpu-profiler/JsNativeJsSample': [SKIP],
   'test-cpu-profiler/JsNativeJsRuntimeJsSample': [SKIP],
   'test-cpu-profiler/JsNative1JsNative2JsSample': [SKIP],
 
   # BUG(3525). Test crashes flakily.
   'test-debug/RecursiveBreakpoints': [PASS, FLAKY],
   'test-debug/RecursiveBreakpointsGlobal': [PASS, FLAKY],
 
+  # Fails sometimes.
+  'test-debug/ProcessDebugMessagesThreaded': [PASS, FLAKY],
+
   ##############################################################################
   # TurboFan compiler failures.
 
   # Some tests are just too slow to run for now.
   'test-api/Threading*': [PASS, NO_VARIANTS],
   'test-heap/IncrementalMarkingStepMakesBigProgressWithLargeObjects': [PASS, NO_VARIANTS],
   'test-heap-profiler/ManyLocalsInSharedContext': [PASS, NO_VARIANTS],
   'test-debug/ThreadedDebugging': [PASS, NO_VARIANTS],
   'test-debug/DebugBreakLoop': [PASS, NO_VARIANTS],
   # BUG(3742).
@@ skipping 306 matching lines @@
   'test-api/ExternalArrays': [SKIP],
 
   # issues to be investigated based on latest uplevel
   'test-run-machops/RunWord64EqualInBranchP': [SKIP],
 
   # will not pass until we agree/implement changes to serializce.cc
   'test-serialize/SerializeInternalReference': [SKIP],
 
 }],  # 'arch == ppc64 and simulator_run == True'
 ]