Simplify ChildProcessLauncher

Remove the refcounted internal state object (nested class 'Context').
This refactor makes it more obvious what happens on what thread,
and avoids the need to pass refptrs around.

TBR=bradnelson@chromium.org
BUG=469248

Review URL: https://codereview.chromium.org/1022703007

Cr-Commit-Position: refs/heads/master@{#322695}
11 files changed