Making MultiProcess tests use the ChildProcessService in base.

Now that ChildProcessService has been moved to base, the multiprocess
tests can use it instead of their duplicated version.
Also simplifying the way we retrieve the service's main method result.
Instead of the client polling, the service is now pushing the result to
the client.

Bug: 702316
Change-Id: If9cae5f0a6007a370b5d1788280cc2ba3b523a06
Reviewed-on: https://chromium-review.googlesource.com/545266
Commit-Queue: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482655}
12 files changed