Remove unneeded extern keywords in tools/gn.

Change-Id: I27262f6dbcb146f9c0939d03e831b750b8978c8f
Reviewed-on: https://chromium-review.googlesource.com/765048
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#516029}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e03b10a94417567821abacbb21c98411bcc64d08