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This patch fixes a crash in which the smoothness notifier triggers
a RenewTreePriority call after LTHI disappears. In that case,
we cancel the callback when the LTHI is gone.

R=reveman, brianderson
BUG=411211

Review URL: https://codereview.chromium.org/548423002

Cr-Commit-Position: refs/heads/master@{#293835}