From jdonnelly during review

Most of the methods in this class [AutocompleteProviderClient] use an iOS-specific class (BrowserState) and thus can't share code with the desktop implementation. But these two new methods and at least a few existing ones (IsPersonalizedUrlDataCollectionActive(), DeleteMatchingURLsForKeywordFromHistory(), Classify()) are identical to the implementations in ChromeAutocompleteProviderClient.

AutocompleteProviderClient is currently pure virtual but I don't think there's any reason it needs to be? Not in this CL, but would you mind investigating whether the common definitions could be put in a new .cc file, similar to how some of our other cross-platform definitions work (e.g. components/omnibox/browser/omnibox_edit_controller.cc). We could also move the empty definitions below (last 3 methods in this file) to defaults in the .cc file, again similar to OmniboxEditController.