GitLab

Since provided file systems are no longer always backed by
extensions, we can no longer generate icon urls for all of them
in Files app.

This CL moves the icon url generation logic to implementations
of ProviderInterface.

Test: TBD.
Bug: 796800
Change-Id: I40ea58d608273f7f9b623e3bd2a1814f285c5d00
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Reviewed-on: https://chromium-review.googlesource.com/871432
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530075}