Remove the senderId argument to PushManager.register().

The sender Id can now be provided by developers using the Web Manifest
API. Stop accepting it as part of the API. (The Web Push API has not
shipped yet, so this should not impact developers.)

Specification:
  http://w3c.github.io/push-api/#pushregistrationmanager-interface

BUG=424601

Review URL: https://codereview.chromium.org/661093002

git-svn-id: svn://svn.chromium.org/blink/trunk@184414 bbb929c8-8fbe-4397-9dbb-9b2b20218538