Add a fuzzer for gn's parser.

BUG=
NOTRY=true

Review-Url: https://codereview.chromium.org/2275683002
Cr-Original-Commit-Position: refs/heads/master@{#414106}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e47a47636163bd171399c6426e834837a52b75b7