Service Manager: Fix client_process capability enforcement

Changes Service Manager to actually enforce the client_process
capability when handling StartServiceWithProcess requests from
service instances. This enforcement regressed in r444496.

BUG=740710
R=jcivelli@chromium.org

Change-Id: I6f0e6880f5c48a13d1b769ae6903ef96746f9030
Reviewed-on: https://chromium-review.googlesource.com/566479
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485809}
6 files changed