Prefetch all WPR archives used by system_health_smoke_test

This also enables system_health_smoke_test on android.chromium bot (Android N5X Swarm Builder).

BUG=700426

Review-Url: https://codereview.chromium.org/2752033002
Cr-Commit-Position: refs/heads/master@{#457813}

testing/buildbot/chromium.android.json

@@ -1183,8 +1183,7 @@
       {
         "args": [
           "--browser=android-chromium",
-          "--device=android",
-          "--skip=benchmarks.system_health_smoke_test.SystemHealthBenchmarkSmokeTest.*"
+          "--device=android"
         ],
         "isolate_name": "telemetry_perf_unittests",
         "name": "telemetry_perf_unittests",

tools/perf/benchmarks/system_health_smoke_test.py

@@ -155,8 +155,14 @@ def load_tests(loader, standard_tests, pattern):
     # Since none of our system health benchmarks creates stories based on
     # command line options, it should be ok to pass options=None to
     # CreateStorySet.
-    for story_to_smoke_test in (
-        benchmark_class().CreateStorySet(options=None).stories):
+    stories_set = benchmark_class().CreateStorySet(options=None)
+
+    # Prefetch WPR archive needed by the stories set to avoid race condition
+    # when feching them when tests are run in parallel.
+    # See crbug.com/700426 for more details.
+    stories_set.wpr_archive_info.DownloadArchivesIfNeeded()
+
+    for story_to_smoke_test in stories_set.stories:
       suite.addTest(
           _GenerateSmokeTestCase(benchmark_class, story_to_smoke_test))