window.Notification.permission directly checks the
HostContentSettingsMap for notification permission. This means that if
the permission is embargoed, it will incorrectly return "default"
instead of "denied".

This CL refactors PermissionDecisionAutoBlocker::GetEmbargoResult to be
a static method that can be called with a HostContentSettingsMap
argument. This allows the PlatformNotificationServiceImpl (which is
running on the IO thread) to check the embargo status of the
notification permission and return the correct value.

The long-term fix here is that PermissionManager::GetPermissionStatus
must be made thread safe so that there is one and only one way to
correctly query permission status from any thread.

BUG=730273

Review-Url: https://codereview.chromium.org/2926773002
Cr-Commit-Position: refs/heads/master@{#477856}