GitLab

* Add ContextImpl service.
* Add FrameImpl service.
* Add support for exiting after final disconnect to ContextProvider.
* Add Context process startup logic to main.cc.
* Rename webrunner_main.cc to web_context_service_main.cc.

Bug: 852145
Change-Id: I87228259472cc0ef1044ea3e00f94de9a0f61990
Reviewed-on: https://chromium-review.googlesource.com/1114321
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574445}