Remove PushPermissionClient.

I found that I still need a multi purpose dispatcher for the Push API. I'll re-implement the permission request in a followup CL.

BUG=389194

Review URL: https://codereview.chromium.org/728443002

git-svn-id: svn://svn.chromium.org/blink/trunk@185297 bbb929c8-8fbe-4397-9dbb-9b2b20218538