Check PrefixSet results for sorting and duplication.

Check PrefixSet results for sorting and duplication.

Check whether PrefixSet::GetPrefixes() returns results that are
unsorted or contain duplicates, and indicate how many items are unique
to the restored vector (or aren't in the restored vector).

BUG=71832
TEST=histograms

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=78807

[Scott Hess - ex-Googler, 2011-03-18 23:38:46]

Scratched together a few minutes to rebase the other changes in.  These were the
boundary checks I could think of - doesn't seem like they're likely to fire, but
I couldn't think of any further to take them, without having a better idea of
whether it's more likely to be off-by-a-little or off-by-a-lot.

[lzheng, 2011-03-19 00:43:06]

LGTM.

I started to wonder if there is a bug in STL. Hope we could get more info after
this change.