CrostiniSharePath small fixes

Use const ref for strings in all functions.
Fix set_mount_event_seneschal_callback_for_testing to match style guide.

Change were made as part of adding FileWatchers to unshare when
paths are deleted but have been split into separate CL.

Bug: 933564
Change-Id: I250c22449b1d2d51c4333ac4c4b13f840f3eb421
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530211
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641817}
3 files changed