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This code significantly simplies the logic for matching.

Indeed, the old logic was buggy because multiple prefix matches could
results in explosive growth of the list of matches (even if only one
bookmark matches).  Reviewer, I would avoid reading and trying to
understand what CombineMatches() and the code that calls it is currently
broken.  See the second test case I added.  Before this
change, that test never terminates (in the time I waited) and explodes
the machine's memory.  Luckily this was rare; there are few reports on
crbug.

It's possible this new code is slower or faster than before (when the
old code behaved correctly).  I plan to look at the
Omnibox.Providertime.Bookmarks numbers before and after this change.

TBR=sky for owners stamp; review it if you feel like

BUG=450850,434604

Review URL: https://codereview.chromium.org/882823004

Cr-Commit-Position: refs/heads/master@{#313574}