GitLab

- delete prepopulated search engines from user prefs
- delete the default search engine from user prefs
- reset unmanaged and non-extension URLs to prepopulated list. 

New unit tests:
- TemplateURLPrepopulateDataTest.ClearProvidersFromPrefs
- TemplateURLServiceTest.ResetNonExtensionURLs
- TemplateURLServiceTest.ResetURLsWithManagedDefault

BUG=235037,244291

Review URL: https://chromiumcodereview.appspot.com/15572002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202869 0039d316-1c4b-4281-b951-d872f2087c98