Mojoify control messages for child processes.

Interface ChildControl is implemented by threads in Child processes, e.g.
ChildThreadImpl, RenderThreadImpl.

The ChildControl methods seemed like a good fit for interface Child in
child.mojom, but that interface is empty and implemented by
content::ServiceManagerConnectionImpl::IOThreadContext, not
ChildThreadImpl.

Bug: 775817
Change-Id: Ica5f0e680366c25356f1db7cb3d7091c4305fdba
Reviewed-on: https://chromium-review.googlesource.com/734500
Commit-Queue: Nicholas Verne <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: John Abd-El-Malek <[email protected]>
Reviewed-by: Ken Rockot <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Sam McNally <[email protected]>
Cr-Commit-Position: refs/heads/master@{#512115}
18 files changed