Work around multiprocessing limits on Windows (#34489)

Python 3 may hang when starting more than about 60 child processes using
the multiprocessing module. Clamping to 56 gives some margin for error
and recognizes that going too high hits diminishing returns (due to
startup costs).

More information can be found in https://crbug.com/1190269 and
https://crbug.com/1336854.

The specific issue found is that "git cl presubmit --force --all" in a
Chromium repo on a machine with more than 60 cores will hang when it
gets to the step that runs lint.py.

tools/lint/lint.py

@@ -1025,6 +1025,11 @@ def lint(repo_root, paths, output_format, ignore_glob=None, github_checks_output
 
     if jobs == 0:
         jobs = multiprocessing.cpu_count()
+        if sys.platform == 'win32':
+            # Using too many child processes in Python 3 hits either hangs or a
+            # ValueError exception, and, has diminishing returns. Clamp to 56 to
+            # give margin for error.
+            jobs = min(jobs, 56)
 
     with open(os.path.join(repo_root, "lint.ignore")) as f:
         ignorelist, skipped_files = parse_ignorelist(f)