Remove post_launch_on_client_thread_called

This is a follow up to refs/heads/master@{#465155}. Android now always
calls OnChildProcessStarted on the launcher thread, which means the
complexity to deal with the call directly happening on the client thread
can be removed.

BUG=689758

Review-Url: https://codereview.chromium.org/2843063002
Cr-Commit-Position: refs/heads/master@{#467463}
3 files changed