crash-reporter: Fix overzealous crash purging in guest mode.

Previously, if crash_sender was invoked during a guest session all crashes that
could be accessed (i.e. that were stored outside of user directories) were
deleted.  This is fixed by re-ordering some of the conditions in send_crashes().

The problem had been introduced by 57f6efa4:
https://chromium-review.googlesource.com/200060

BUG=chromium:393334
TEST=unit tests passed
TEST=try success on amd64-generic-full

Change-Id: I931665891eca8e645135f7f08c15b3086fcb7d73
Reviewed-on: https://chromium-review.googlesource.com/213470


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Thiemo Nagel <tnagel@google.com>
Tested-by: Thiemo Nagel <tnagel@google.com>