Replace dash by underscore in the command line argument before absl flag parsing.

The expected behavior is to have something similar than python:
https://docs.python.org/dev/library/argparse.html#dest:
"Any internal - characters will be converted to _ characters to make sure the string is a valid attribute name".

This allows to catch chromium arguments like 'isolated-script-test-output' that previously needed some preprocessing done for example in flags_compatibility.py.

This CL also fixes a fuchsia specific issue where the test runner needs a 'isolated-script-test-output' argument but then pass the argument to WebRTC that expects a 'isolated_script_test_output' argument. Thus calling flags_compatibility before the test_runner fails and there is not much room to change the argument in between the test runner and the test.

Change-Id: I48a591743fa50484a0ec584a3f9e97d9e0fd25ef
Bug: webrtc:14694
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/284541
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38707}

tools_webrtc/gtest_parallel_wrapper_test.py

@@ -146,7 +146,7 @@ class GtestParallelWrapperTest(unittest.TestCase):
           '--output_dir=' + output_dir, '--dump_json_test_results=SOME_DIR',
           'some_test', '--', '--test_artifacts_dir=' + expected_artifacts_dir,
           '--some_flag=some_value', '--another_flag',
-          '--isolated_script_test_perf_output=SOME_OTHER_DIR', '--foo=bar',
+          '--isolated-script-test-perf-output=SOME_OTHER_DIR', '--foo=bar',
           '--baz'
       ])
       self.assertEqual(result.gtest_parallel_args, expected)