[telemetry] Fix benchmark smoke unittest _disabled_strings logic, take 2.

tools/perf/benchmarks/benchmark_smoke_unittest.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Run the first page of one benchmark for every module.
 
 Only benchmarks that have a composable measurement are included.
 Ideally this test would be comprehensive, however, running one page
 of every benchmark would run impractically long.
 """
@@ skipping 88 matching lines @@
 
     method = SmokeTestGenerator(benchmark)
 
     # Make sure any decorators are propagated from the original declaration.
     # (access to protected members) pylint: disable=W0212
     # TODO(dpranke): Since we only pick the first test from every class
     # (above), if that test is disabled, we'll end up not running *any*
     # test from the class. We should probably discover all of the tests
     # in a class, and then throw the ones we don't need away instead.
     if hasattr(benchmark, '_enabled_strings'):
-      if hasattr(method, '_enabled_strings'):
+      if (hasattr(method, '_enabled_strings') and
+          method._enabled_strings and benchmark._enabled_strings):
         method._enabled_strings += benchmark._enabled_strings
-      else:
+      elif not hasattr(method, '_enabled_strings') or method._enabled_strings:
         method._enabled_strings = benchmark._enabled_strings
     if hasattr(benchmark, '_disabled_strings'):
-      if hasattr(method, '_disabled_strings'):
+      if (hasattr(method, '_disabled_strings') and
+          method._disabled_strings and benchmark._disabled_strings):
         method._disabled_strings += benchmark._disabled_strings
-      else:
+      elif not hasattr(method, '_disabled_strings') or method._disabled_strings:
         method._disabled_strings = benchmark._disabled_strings
     setattr(BenchmarkSmokeTest, benchmark.Name(), method)
 
     suite.addTest(BenchmarkSmokeTest(benchmark.Name()))
 
   return suite