Move more process-killing logic into *Job

There's no reason for the SessionManagerService to hold logic for
waiting on and aborting un-term-able child processes. It belongs
more properly behind the ChildJob abstraction.

BUG=chromium:330882
TEST=unit

Change-Id: I1ec51b8b113b7203990eee313cbfe1553dd43325
Reviewed-on: https://chromium-review.googlesource.com/183898
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Daniel Erat <derat@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>