Trusted plugin: Break up InitCommunication().

ServiceRuntime::InitCommunication is a large method that does several things:
  - Sets up a command channel.
  - Loads a module (nexe).
  - Sets up a reverse channel.
  - Starts a module (nexe).

This change breaks this up into smaller methods. As part of the trusted plugin
refactor, we'll probably move setting up the reverse channel before loading the
module, and join loading and starting the module into one step.

This change should make the changes to use Chromium IPC easier to follow.

BUG=333950

Review URL: https://codereview.chromium.org/133073005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245555 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245555