Reland "Move GPU and Plugin processes off Service Manager"

This is a reland of bdc395526e7b24cc7c0d522c28dce71a9c1b3670

Original change's description:
> Move GPU and Plugin processes off Service Manager
>
> This introduces a new mode for Browser/ChildProcessHost where the
> primordial pipe is a simple content.mojom.ChildProcess pipe. This will
> become the new default mode of operation for child processes, and
> eventually will be the only mode of operation once NaCl processes are
> gone.
>
> GPU and plugin processes are ported to this mode here since both
> require only minimal changes. This means that "content_gpu" and
> "content_plugin" services no longer exist.
>
> Bug: 977637
> Change-Id: I39c5d2284bd84246663d5f0eb4cbab3b3483d798
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918326
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Commit-Queue: Ken Rockot <rockot@google.com>
> Cr-Commit-Position: refs/heads/master@{#717857}

NOPRESUBMIT=true

Bug: 977637
Change-Id: Iaefab7950d7f0437b05417a34ff2b1db73576f0b
Tbr: avi@chromium.org
Tbr: rsesek@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948907
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#721374}
18 files changed