Start ServiceManger before creating BrowserMainLoop.

This CL instantiates the ServiceManagerContext before creating
the BrowserMainRunner. It splits the startup path into two,
with/without starting the full browser. The changes are implemented
behind a flag "allow-start-service-manager-only".

Bug: 846846,729596
Change-Id: I3584db0d89a10e59d6041e0f4412aaffdc840568
Reviewed-on: https://chromium-review.googlesource.com/c/1113802
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603092}
9 files changed