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It is a feature that allows to collect data from HashTables (e.g. rehashes,
reads, number of collisions, etc).

This change does:
- fixes the functionality for general HashTable stats
- also fixes for individual HashTables

And introduces a public member function (guarded behind a macro)
called DumpStats() that allows to dump the stats from HashMap.

Bug: 735663
Change-Id: I86f826c3d23f02f0d06dba7e0a5a157601bbd4ce
Reviewed-on: https://chromium-review.googlesource.com/556919
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488777}