[Telemetry] Always set return code.

This fixes the exception:

  File "/b/s/w/ir/testing/scripts/run_telemetry_benchmark_as_googletest.py", line 176, in run_benchmark
    if rc == 0:
UnboundLocalError: local variable 'rc' referenced before assignment

TBR=nednguyen

Bug: 891332
Change-Id: I81b8502f8f95bf8db070d8ccc062a343b062e880
Reviewed-on: https://chromium-review.googlesource.com/c/1258978
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596316}

testing/scripts/run_telemetry_benchmark_as_googletest.py

@@ -126,6 +126,7 @@ def run_benchmark(args, rest_args, histogram_results):
     cmd_args = cmd_args + [
       '--story-filter=' + filter_regex
     ]
+  rc = 1  # Set default returncode in case there is an exception.
   try:
     cmd = [sys.executable] + cmd_args + [
       '--output-dir', tempfile_dir,