Make benchmark_enabled_map key look-up returns False if key not found

NOTRY=true  # this does not have unittest
TBR=eyaich@chromium.org

Bug: 859251
Change-Id: Ic3da603cb16b5d41ecc2368ff7492735ec26e147
Reviewed-on: https://chromium-review.googlesource.com/1121519Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#571729}

tools/perf/process_perf_results.py

@@ -344,7 +344,7 @@ def _handle_perf_results(
     upload_failure = False
     with oauth_api.with_access_token(service_account_file) as oauth_file:
       for benchmark_name, directories in benchmark_directory_map.iteritems():
-        if not benchmark_enabled_map[benchmark_name]:
+        if not benchmark_enabled_map.get(benchmark_name, False):
           continue
         upload_begin_time = time.time()
         # There are potentially multiple directores with results, re-write and