Delete ServiceContextRefFactory

This is fully replaced by ServiceKeepalive, with the last few uses
being converted here.

service_context_ref.h is left in place for now, defining
ServiceContextRef as type alias for ServiceKeepaliveRef. Follow-up
changes will do the mechanical rewriting of these symbols.

TBR=xiaohuic@chromium.org

Bug: 891780
Change-Id: I5798bc0979c73fb4a9a977c81b71662a5cb8b2ad
Reviewed-on: https://chromium-review.googlesource.com/c/1359693
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#614223}
21 files changed