Factor out inner-classes out of ChildProcessLauncher.

Factoring out ChildProcessAllocator and SpawnData out of
ChildProcessLauncher.
Also:
- simplifying ChildProcessAllocator by lazy allocating on access rather
  than explicitly.
- removing forWarmUp from SpawnData has it can be handled as a parameter
  passed around in ChildProcessLauncher.

BUG=702316

Review-Url: https://codereview.chromium.org/2795113003
Cr-Commit-Position: refs/heads/master@{#463916}
7 files changed