Remove unused DeferrableOneShotTimer.
Remove unused DeferrableOneShotTimer.

This timer has been deferred... permanently. Because apparently nobody
uses it. This is a bit of cleanup in preparation for Issue 331912.

BUG=331912

Review URL: https://codereview.chromium.org/847933002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@188279 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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