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This uses even less memory (a few hundred bytes) by sacrificing some
worst case performance (in the order of a microsecond per query) and
maintainability.

Estimated memory savings will be 9-12 KB. (3 KB per QuerySyncManager).

BUG=485536
R=reveman@chromium.org,jbauman@chromium.org

Review URL: https://codereview.chromium.org/1129253006

Cr-Commit-Position: refs/heads/master@{#330933}