Start pulling away notification logic from ChromeContentBrowserClient.

This moves a bunch of notification permission logic (specifically for
extensions) from ChromeContentBrowserClient to a new object, the
PlatformNotificationService, which is a singleton in the browser process.

BUG=439950

Review URL: https://codereview.chromium.org/774573003

Cr-Commit-Position: refs/heads/master@{#307694}
14 files changed