Restrict `file:` stylesheets to `.css` extensions.

Bug: 788936
Change-Id: Icd4e22745561d57c691fd0f0e75bb8c3e4a59303
Reviewed-on: https://chromium-review.googlesource.com/795717
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: nainar <nainar@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520610}
19 files changed