[Fuchsia] Add constants for /data and /svc

/data and /svc paths were hardcoded in a few places. Added constants
in //base/fuchsia/file_utils.h and updates //base and //fuchsia to use
them.

Change-Id: Id700408494c24cb5986b06231682c9d83058bc08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1511708
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639723}
9 files changed