HUP Experimental Scoring framework.

High level summary:
(a) Introduce a new set of Finch variation params and use it in the Omnibox bundled experiment
(b) Group together HUP scoring params in a HUPScoringParams struct and initialize it based on the new experiment params
(c) Modify HUP scoring by applying optional demotion 

This change is fairly safe because of the following reasons:
(1) It is disabled by default
(2) It never changes the relative order of HUP matches
(3) It can only demote a HUP match
(4) It is fairly isolated and unobtrusive

BUG=295756
TESTS=OmniboxFieldTrialTest,HistoryURLProviderTest

Review URL: https://codereview.chromium.org/23707058

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242569 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242569