Merge search engines sync data type with Preferences. Sync the default search provider. Add some defensive measures to prevent deletion of the default search engine or unnecessarily uniquifying keywords.

TEST=Ensure that the default search provider syncs when the Preferences sync data type is enabled. Ensure that the normal search engine syncing changes (add, update, delete) all work.
BUG=15548
Review URL: http://codereview.chromium.org/8334030

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@109882 0039d316-1c4b-4281-b951-d872f2087c98
29 files changed