Remove trivial use of --isolate-script-test-perf-output flag.

The above flag is currently always passed by the recipe to
isolated_script tests. Nothing uses it for anything substantial,
but I forgot when I tried to remove it in https://crrev.com/c/2427305
that there is one trivial use of it (to reconstruct a command line).

This CL removes that one trivial use, so that I can re-land the
original build-side recipe change and remove the flag.

Bug: 1127545
Change-Id: I11b861be35be43f193be24117073a69906c5a73a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429866
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#810743}

testing/scripts/run_rendering_benchmark_with_gated_performance.py

@@ -142,8 +142,6 @@ class RenderingRepresentativePerfTest(object):
 
     re_run_test_output = os.path.join(re_run_output_dir,
       os.path.basename(self.options.isolated_script_test_output))
-    re_run_test_perf_output = os.path.join(re_run_output_dir,
-      os.path.basename(self.options.isolated_script_test_perf_output))
 
     self.set_platform_specific_attributes()
 
@@ -165,8 +163,7 @@ class RenderingRepresentativePerfTest(object):
     self.args.extend(['--story-tag-filter', self.story_tag])
 
     self.re_run_args = replace_arg_values(list(sys.argv), [
-      ('--isolated-script-test-output', re_run_test_output),
-      ('--isolated-script-test-perf-output', re_run_test_perf_output)])
+      ('--isolated-script-test-output', re_run_test_output)])
 
   def parse_csv_results(self, csv_obj):
     """ Parses the raw CSV data