Push API: delete separate requestPermission path.
Push API: delete separate requestPermission path.

Having separate calls to request permission leads to more complexity in Blink than expected.

The new plan is here:
https://www.lucidchart.com/documents/view/b710dc56-4cd2-4bfe-8729-991dfdca137b

Depends on https://codereview.chromium.org/768563003/

BUG=389194

Review URL: https://codereview.chromium.org/764833002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186134 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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