[Files app] Fix device_path_ member to be a value not a reference

RemovableTestVolume should own a FilePath as value not as reference,
otherwise it isn't guaranteed that the value is valid.

This fixes ASAN bot:
https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtLzQ5OWE0NmMyNzQ0MTVjYmFlM2RjMDIyZmQxM2Q0ZWU2YjJjNmJkZGIM
crrev.com/c/1474699

Bug: 918795
Change-Id: I269d3effb651947fc18985736d105d7bc9326c7d
Reviewed-on: https://chromium-review.googlesource.com/c/1474959
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632991}
1 file changed