Implement PermissionService::GetNextPermissionChange.

This is adding two methods to start and stop observing permission
changes from content::PermissionManager.

BUG=430238
TBR=gunsch@chromium.org (for chromecast/ boilerplate)

Review URL: https://codereview.chromium.org/990303002

Cr-Commit-Position: refs/heads/master@{#323210}
15 files changed