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Store default search provider data in dictionary pref, and add DefaultSearchManager class to handle the reading and writing of this pref (DefaultSearchManager will eventually all default search related concerns).

This pref will be used to persist data about user-selected 
default search providers. In future CLs, support will be added for default search providers set by policy and sync.


BUG=365762

Review URL: https://codereview.chromium.org/229763005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266479 0039d316-1c4b-4281-b951-d872f2087c98