Push API: Add PushMessagingService::PushEndpoint (assume it's constant)
For https://crbug.com/429212 we'll need to store the currently
registered push endpoint + registration ID pairs. Since push endpoint
is a constant for all known implementers of Chromium's content layer,
we can add a no-arguments accessor to the content layer which returns
it directly. This will let us save storage space once we store these
pairs.
BUG=429212,401422
Review URL: https://codereview.chromium.org/
762003003
Cr-Commit-Position: refs/heads/master@{#306224}