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It's empty on Blink side now. Rather than implementing it or leave it empty,
just stop calling it. In Chromium, content::NotificationProvider never goes
away.

BUG=none

Review URL: https://codereview.chromium.org/36073007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233519 0039d316-1c4b-4281-b951-d872f2087c98