Add the possibility to output a json gtest output to the perf tests.

We use the Chromium existing flag isolated_script_test_output that we translate into gtest_output.
This is because the Chromium flag has the same purpose as gtest_output and is already provided in the recipe modules.

No-Presubmit: True
Bug: b/197492097
Change-Id: Ia432a85b0e0ab32008b39ffe751d11aefb9b24ea
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251041
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#35937}
diff --git a/tools_webrtc/flags_compatibility.py b/tools_webrtc/flags_compatibility.py
index caad537..bdafefb 100755
--- a/tools_webrtc/flags_compatibility.py
+++ b/tools_webrtc/flags_compatibility.py
@@ -22,9 +22,7 @@
 
   test_command = _ForcePythonInterpreter(unrecognized_args)
   if args.isolated_script_test_output:
-    test_command += [
-        '--isolated_script_test_output', args.isolated_script_test_output
-    ]
+    test_command += ['--gtest_output=json:' + args.isolated_script_test_output]
   if args.isolated_script_test_perf_output:
     test_command += [
         '--isolated_script_test_perf_output=' +