Remove the OnError method from notification delegates.

OnError was meant to support the "onerror" event for Web Notifications,
but it has no call sites. There are a few future use-cases for this event,
but those don't concern non-Web Notifications so we don't need this plumbing.

BUG=428370

Review URL: https://codereview.chromium.org/687183004

Cr-Commit-Position: refs/heads/master@{#302248}
25 files changed