Creating a chrome ChildProcessServiceImpl class for WebAPK.

WebApkSandboxedProcessService accesses ChildProcessServiceImpl (which is
in content) through reflection and needs to be exposed to WebAPK.
Introducting a new chrome class that exposes this and forwards to
ChildProcessServiceImpl.
This also prepares for the move of ChildProcessServiceImpl to base where
more work will need to be done when creating a ChildProcessServiceImpl.

BUG=702316

Change-Id: Ib70f72c430a8a48b8ee84db5c6359f4c6f6dddfb
Reviewed-on: https://chromium-review.googlesource.com/528669
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478757}
5 files changed