Clean-up in ChildProcessLauncher.

This is a clean-up follow-up for
https://codereview.chromium.org/2760093002:
- the callback type parameter is replaced with the actual callback
  object.
- the alwaysInForeground parameter is now part of the SpawnData class.
- SpawnData accessors are renamed to follow Java naming conventions.

BUG=702316

Review-Url: https://codereview.chromium.org/2792093002
Cr-Commit-Position: refs/heads/master@{#461798}
2 files changed