[FileSystem] Get rid of some abstraction layers for recently added methods.

Now that FileSystem is onion souped it is easier to just call the mojom
methods directly instead of going through several layers of indirection.

Bug: 787281
Change-Id: Ia92ff56c2de354dcce918537d901a9b8dcef4218
Reviewed-on: https://chromium-review.googlesource.com/c/1244824
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598117}