GitLab

PushMessagingDispatcher is now trying to get the Manifest before
asking the registration to happen. If the gcm_sender_id is set, it will
be used. Otherwise, the flow is unchanged.

This is an considered option in order to no longer require an opaque
property bag to be passed when registering. It might or might not stay
as is.

BUG=414873

Review URL: https://codereview.chromium.org/609393002

Cr-Commit-Position: refs/heads/master@{#297403}