[FileSystem]: Use WeakPtr in FileWriterImpl where appropriate.

Bug: 922677
Change-Id: Icd9466fb40cc54e94ffc612801bd73f2e59338ca
Reviewed-on: https://chromium-review.googlesource.com/c/1416630
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623524}
2 files changed