Move ChildProcessLauncher to base/.

Moving the ChildProcessLauncher class to base.
This will allow the Multiprocess tests (used by mojo/ and ipc/) to
reuse the launching code instead of duplicating it.

TBR=dcheng

Bug: 702316
Change-Id: I8f05ce382bd87fe4f681fa510e1a61eab2d15428
Reviewed-on: https://chromium-review.googlesource.com/559833
Commit-Queue: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Bo Liu <boliu@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485832}
21 files changed