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Instead of iterating over every handle, only iterate over those that have
a deadline set (and hence can expire). This requires tracking which
handles have deadlines.

A better solution would be to use a priority queue to track the closest
deadline. However, it turns out that no-one currently uses deadlines here, so
the size of |deadline_handles_| will always be 0.

BUG=556865

Review URL: https://codereview.chromium.org/1448233002

Cr-Commit-Position: refs/heads/master@{#361291}