Improve presubmit message for QuitWhenIdleClosure().

Also makes this error about deprecated functions into a warning.  The problems
with using these are not so severe they should block uploads.

BUG=none
TEST=none

Change-Id: I94e6cc900e23d8f3371aace9386a80829906083b
Reviewed-on: https://chromium-review.googlesource.com/902448
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534542}