Add support for creating gn files to boilerplate.py script.
Add support for creating gn files to boilerplate.py script.

Add .gn and .gni to the list of know extensions for boilerplate.py
script.

BUG=None

Review URL: https://codereview.chromium.org/1796153003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381205}

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