Stop unfinished speech query from appearing in app list reopen.

This CL fixes an issue where a speech query in the app list, that was
interrupted by the app list closing, would complete when the app list
was reopened.

BUG=440061

Review URL: https://codereview.chromium.org/886353004

Cr-Commit-Position: refs/heads/master@{#314523}

chrome/browser/ui/app_list/start_page_service.cc

@@ -348,6 +348,7 @@ void StartPageService::AppListHidden() {
   if (HotwordService::IsExperimentalHotwordingEnabled() &&
       speech_recognizer_) {
     speech_recognizer_->Stop();
+    speech_recognizer_.reset();
   }
 
 #if defined(OS_CHROMEOS)