cryptohome: Emit a signal when disk space is low.

BUG=b:29342389
TEST=cros_workon_make --board=$BOARD --test cryptohome

Change-Id: I78a33438c29e5de228ea68c8ab7f3738e935f59d
Previous-Reviewed-on: https://chromium-review.googlesource.com/352371
(cherry picked from commit 1c386ee57581c9d2541546317219b1b0a01b71d9)
Reviewed-on: https://chromium-review.googlesource.com/361520
Trybot-Ready: Dan Spaid <dspaid@chromium.org>
Tested-by: Dan Spaid <dspaid@chromium.org>
Reviewed-by: Junichi Uekawa <uekawa@chromium.org>
Commit-Queue: Junichi Uekawa <uekawa@chromium.org>
7 files changed