Fix type issue for the uploader.

This is a forward fix for https://webrtc-review.googlesource.com/c/src/+/255320.
The error is the following:
https://logs.chromium.org/logs/webrtc/buildbucket/cr-buildbucket/8819700229708372817/+/u/low_bandwidth_audio_perf_test_Dashboard_upload/stdout

Bug: webrtc:13806
Change-Id: Ia7d5362d1131d9bab41a9112946a4c84e509d98f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/255382
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#36195}

tools_webrtc/perf/webrtc_dashboard_upload.py

@@ -58,12 +58,10 @@ def _CreateParser():
                       required=True,
                       help='Which dashboard to use.')
   parser.add_argument('--input-results-file',
-                      type=argparse.FileType('rb'),
                       required=True,
                       help='A HistogramSet proto file with output from '
                       'WebRTC tests.')
   parser.add_argument('--output-json-file',
-                      type=argparse.FileType('w'),
                       help='Where to write the output (for debugging).')
   parser.add_argument('--outdir',
                       required=True,