Improved iterator for persistent memory allocator.

The previous "iterator" was just state that was updated
by the main class which is not generally how iterators
operate.  New version creates self-contained objects
that do the "getnext" completely on their own.

Also fixed old bug with incorrect construction parameters
for histograms in test.  It never affected operation but
generates VLOG messages and affects timing tests.

BUG=546019

Review URL: https://codereview.chromium.org/1803253002

Cr-Commit-Position: refs/heads/master@{#385467}
9 files changed