GitLab

The network handler purpose is:

1. Connect to the GCM channel
2. Retrieve the instance_id token and forward it to the registration manager.
3. Forward incoming messages to the message handlers.

Bug: 863893, 801985
Change-Id: I236e5a2374468aaceb69641b05d535a3685d716e
Reviewed-on: https://chromium-review.googlesource.com/1138317
Commit-Queue: Tatiana Gornak <melandory@chromium.org>
Reviewed-by: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576506}