[chrome:omnibox] clear previous outputs on making a new request.

A line was accidentally left out when moving code in the previous cl [1] resulting in outputs from the previous query not being cleared when making a new query when the `Show incomplete results` option was enabled.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1252324

Change-Id: I12f8465898171badff2e6c4f69fcbcaf121f680b
Bug: 891303
Reviewed-on: https://chromium-review.googlesource.com/c/1264815Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597156}

chrome/browser/resources/omnibox/omnibox.js

@@ -435,6 +435,7 @@
      * call handleNewAutocompleteResult as results come in.
      */
     makeRequest() {
+      clearOutput();
       this.progressiveAutocompleteResults = [];
       // Then, call chrome with a five-element list:
       // - first element: the value in the text box