[Extensions] Include mime type in served extension files

Modules enforce strict mime type requirements, but files served through
extension protocols don't specify a mime type. Include it in the
response headers. This enables modules for chrome extensions.

Add a test to exercise basic module use.

Bug: 769012
Change-Id: I1eba64e9194c00b9feb39f5fb0f5576a53d34930
Reviewed-on: https://chromium-review.googlesource.com/685802
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504885}
7 files changed