GitLab

Implements ReadFile for SmbFileSystem. This will allow files to be read
from the files app.

Bug: chromium:757625
Change-Id: I3bae2b3c631ef78941a3f403f31c45c98afdf33a
Reviewed-on: https://chromium-review.googlesource.com/848413Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Allen Vicencio <allenvic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530394}