Clean up standalone service entry points

This gets rid of the old C-linkage entry point for standalone service
binaries, in favor of a cleaner and typesafe C++ entry point which takes
a proper ServiceRequest.

TBR=msw@chromium.org
TBR=jamescook@chromium.org

Bug: 911217
Change-Id: Ide390764db97eaa45806fdb3ca0f3e39a408baa7
Reviewed-on: https://chromium-review.googlesource.com/c/1359475
Commit-Queue: Ken Rockot <rockot@google.com>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#613691}
36 files changed