[content] Introduce ServiceProcessHost

ServiceProcessHost allows Content and its embedders to launch a new
service process for any given mojom service interface.

This CL wires up the support and converts a single test-only service
to use ServiceProcessHost instead of integrating with Service Manager.

Bug: 977637
Change-Id: I63e846bf816c51ba2c557bfdc8b41189a92d2890
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1673922
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#672676}
40 files changed