[GN] Adding missing source type for extension .appex.

Match .appex to wrapper.app-extension

BUG=

Review-Url: https://codereview.chromium.org/2029073002
Cr-Original-Commit-Position: refs/heads/master@{#397247}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4feb71fc34f559ae3bd16a5a9bbb70c492d1c5ac