[Benchmarking] Fix error condition.

https://logs.chromium.org/logs/chrome/buildbucket/cr-buildbucket.appspot.com/8893636477245975664/+/steps/dawn_perf_tests_on_ATI_GPU_on_Mac_on_Mac-10.13.3/0/logs/Merge_script_log/0

Traceback (most recent call last):
  File /b/s/w/ir/cache/builder/src/tools/perf/process_perf_results.py, line 361, in process_perf_results
    configuration_name, build_properties, extra_links, output_results_dir)
  File /b/s/w/ir/cache/builder/src/tools/perf/process_perf_results.py, line 548, in _handle_perf_results
    is_reference, upload_failure=not upload_succeed)
  File /b/s/w/ir/cache/builder/src/tools/perf/process_perf_results.py, line 580, in _write_perf_data_to_logfile
    json.dump(results, output_json_file,
UnboundLocalError: local variable 'results' referenced before assignment

Bug: 1035590
Change-Id: I6d0fd9373f94d6484ce42cf867a50e302ef428d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976294
Auto-Submit: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726502}

tools/perf/process_perf_results.py

@@ -573,21 +573,23 @@ def _write_perf_data_to_logfile(benchmark_name, output_file,
   viewer_url = None
   # logdog file to write perf results to
   if os.path.exists(output_file):
+    results = None
     with open(output_file) as f:
       try:
         results = json.load(f)
       except ValueError:
         print('Error parsing perf results JSON for benchmark  %s' %
               benchmark_name)
-    try:
-      output_json_file = logdog_helper.open_text(benchmark_name)
-      json.dump(results, output_json_file,
-                indent=4, separators=(',', ': '))
-    except ValueError as e:
-      print('ValueError: "%s" while dumping output to logdog' % e)
-    finally:
-      output_json_file.close()
-    viewer_url = output_json_file.get_viewer_url()
+    if results:
+      try:
+        output_json_file = logdog_helper.open_text(benchmark_name)
+        json.dump(results, output_json_file,
+                  indent=4, separators=(',', ': '))
+      except ValueError as e:
+        print('ValueError: "%s" while dumping output to logdog' % e)
+      finally:
+        output_json_file.close()
+      viewer_url = output_json_file.get_viewer_url()
   else:
     print("Perf results JSON file doesn't exist for benchmark %s" %
           benchmark_name)
@@ -600,7 +602,8 @@ def _write_perf_data_to_logfile(benchmark_name, output_file,
   # add links for the perf results and the dashboard url to
   # the logs section of buildbot
   if is_ref:
-    logdog_dict[base_benchmark_name]['perf_results_ref'] = viewer_url
+    if viewer_url:
+      logdog_dict[base_benchmark_name]['perf_results_ref'] = viewer_url
     if upload_failure:
       logdog_dict[base_benchmark_name]['ref_upload_failed'] = 'True'
   else:
@@ -610,7 +613,8 @@ def _write_perf_data_to_logfile(benchmark_name, output_file,
             configuration_name, RESULTS_URL,
             build_properties['got_revision_cp'],
             _GetMachineGroup(build_properties)))
-    logdog_dict[base_benchmark_name]['perf_results'] = viewer_url
+    if viewer_url:
+      logdog_dict[base_benchmark_name]['perf_results'] = viewer_url
     if upload_failure:
       logdog_dict[base_benchmark_name]['upload_failed'] = 'True'