Delete the unused WebPushPermissionCallback [3/3]

Patch dependency:
[1] Blink: https://crrev.com/702003004/
[2] Chromium: https://crrev.com/690833004/
[3] Blink: https://crrev.com/708543002/ (THIS PATCH)

BUG=430527

Review URL: https://codereview.chromium.org/708543002

git-svn-id: svn://svn.chromium.org/blink/trunk@184919 bbb929c8-8fbe-4397-9dbb-9b2b20218538