Re-enable telemetry_perf_unittest's BenchmarkSmokeTests on Chrome OS.

According to the comment in the code, disabling this test disables all
benchmark smoke checks in telemetry_perf_unittest. That seems like a
critical set of tests to lose coverage for. So this re-enables them.

Also increases the shard count of telemetry_perf_unittest's on the
chromebook tester since this increases its runtime significantly.

Bug: 1041328

Change-Id: I849a04c57c8f9ac4eb687e93b85322b67cb47cfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032325Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738751}

testing/buildbot/chromium.chromiumos.json

@@ -774,7 +774,8 @@
               "pool": "chrome-cros-dut"
             }
           ],
-          "idempotent": false
+          "idempotent": false,
+          "shards": 2
         },
         "test_target": "//chrome/test:telemetry_perf_unittests",
         "trigger_script": {

testing/buildbot/test_suites.pyl

@@ -488,6 +488,7 @@
         ],
         'swarming': {
           'idempotent': False,  # https://crbug.com/549140
+          'shards': 2,
         },
       },
       'telemetry_unittests': {

tools/perf/benchmarks/benchmark_smoke_unittest.py

@@ -54,7 +54,6 @@ def SmokeTestGenerator(benchmark_class, num_pages=1):
   # failing or flaky benchmark would disable a much wider swath of coverage
   # than is usually intended. Instead, if a particular benchmark is failing,
   # disable it in tools/perf/benchmarks/*.
-  @decorators.Disabled('chromeos')  # crbug.com/351114
   @decorators.Disabled('android')  # crbug.com/641934
   def BenchmarkSmokeTest(self):
     # Some benchmarks are running multiple iterations