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This is necessary because GN's exec_script only runs Python scripts.

This was requested by Nico in https://codereview.chromium.org/971593003/.

BUG=462972
TEST=run ./tools/clang/scripts/plugin_flags.py and compare the output
with the bash version. They should be exactly the same.
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/966723003

Cr-Commit-Position: refs/heads/master@{#318924}