Record zero-suggest selections as empty input.

This makes typed length metric in omnibox event more useful -- we can
tell zero-suggest entries by empty selected input instead of recording
the URL lengths in that case.

BUG=529892

Review URL: https://codereview.chromium.org/1395613006

Cr-Commit-Position: refs/heads/master@{#353948}

components/omnibox/browser/omnibox_edit_model.cc

@@ -676,7 +676,7 @@ void OmniboxEditModel::OpenMatch(AutocompleteMatch match,
   AutocompleteResult fake_single_entry_result;
   fake_single_entry_result.AppendMatches(input_, fake_single_entry_matches);
   OmniboxLog log(
-      input_text,
+      input_.from_omnibox_focus() ? base::string16() : input_text,
       just_deleted_text_,
       input_.type(),
       popup_model()->IsOpen(),