Only check for json-test-results output when the benchmark is enabled

If a benchmark is disabled but still run as a step, FormatDisabled is
called on the output_formatter instead of Format. In the case of
json-test-results, this means that no json will be output by the test
(since there is no empty version of json-test-results).

BUG=chromium:751346

NOTRY=true  # This script is not run on trybot/CQ, hence should be safe to notry

Change-Id: I4f19eeab40c1b6976e0f6c92cd5b7fff8ee17e2e
Reviewed-on: https://chromium-review.googlesource.com/598729Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ashley Enstad <ashleymarie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491622}

testing/scripts/run_telemetry_benchmark_as_googletest.py

@@ -79,15 +79,14 @@ def main():
       with open(chart_tempfile_name) as f:
         chartresults = json.load(f)
 
-    # test-results.json is the file name output by telemetry when the
-    # json-test-results format is included
-    tempfile_name = os.path.join(tempfile_dir, 'test-results.json')
-    with open(tempfile_name) as f:
-      json_test_results = json.load(f)
-
-    # Determine if this was a disabled benchmark that was run
+    # Skip if this was a disabled benchmark that was run
     if (not chartjson_results_present or
        (chartjson_results_present and chartresults.get('enabled', True))):
+      # test-results.json is the file name output by telemetry when the
+      # json-test-results format is included
+      tempfile_name = os.path.join(tempfile_dir, 'test-results.json')
+      with open(tempfile_name) as f:
+        json_test_results = json.load(f)
       num_failures = json_test_results['num_failures_by_type'].get('FAIL', 0)
       valid = bool(rc == 0 or num_failures != 0)