Don't try to create a verbatim match with an empty input.

components/omnibox/browser/verbatim_match.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_OMNIBOX_BROWSER_VERBATIM_MATCH_H_
 #define COMPONENTS_OMNIBOX_BROWSER_VERBATIM_MATCH_H_
 
 #include "base/strings/string16.h"
 #include "components/omnibox/browser/autocomplete_input.h"
 #include "url/gurl.h"
 
 struct AutocompleteMatch;
 class AutocompleteProviderClient;
 class HistoryURLProvider;
 
 // Returns a verbatim match for input.text() with a relevance of
 // |verbatim_relevance|. If |verbatim_relevance| is negative then a default
-// value is used.  If the desired |destination_url| is already known and a
+// value is used. If the desired |destination_url| is already known and a
 // |history_url_provider| is also provided, this may be more efficient (see
 // implementation for details) than the default code path.
+// input.text() must not be empty.
 AutocompleteMatch VerbatimMatchForURL(
     AutocompleteProviderClient* client,
     const AutocompleteInput& input,
     const GURL& destination_url,
     HistoryURLProvider* history_url_provider,
     int verbatim_relevance);
 
 #endif  // COMPONENTS_OMNIBOX_BROWSER_VERBATIM_MATCH_H_