Fix use-after-free in FileReaderLoader.

Anything that calls out to client_ can cause FileReaderLoader to be
destroyed, so make sure to check for that situation.

Bug: 835639
Change-Id: I57533d41b7118c06da17abec28bbf301e1f50646
Reviewed-on: https://chromium-review.googlesource.com/1024450
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552807}