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The only thing we need to know per QuerySync block is its
index in the block sequence. Everything else can be
calculated from that if we just store a few numbers in
the Bucket objects.

Total memory reduction is 92%.

This is alt #2 in bug 485536.

BUG=485536

Review URL: https://codereview.chromium.org/1131913004

Cr-Commit-Position: refs/heads/master@{#330334}