Change ITestClient to be similar to IChildProcessService.

Preparing the MultiprocessTestClientLauncher for transitioning to the
new base ChildProcessLauncher. More specifically, making ITestClient
similar to IChildProcessService. This requires moving extra methods to
a new interface ITestController, and creating a callback interface
ITestCallback that lets the service sets the ITestController interface
on the client.

BUG=702316

Review-Url: https://codereview.chromium.org/2781913002
Cr-Commit-Position: refs/heads/master@{#462683}
7 files changed