Do not use LoadCommittedDetails.item in VoiceSearchNavigationTabHelper.

NavigationItemCommitted is deprecated in favour of DidFinishNavigation.
This change will simplify transition from NavigationItemCommitted to
DidFinishNavigation, because DidFinishNavigation does not pass
NavigationItem as argument.

Bug: 720786
Change-Id: I758d7220c2d600bc663d8c55eeb49b5c54e619ff
Reviewed-on: https://chromium-review.googlesource.com/c/1356770
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613330}

ios/chrome/browser/voice/voice_search_navigations_tab_helper.mm

@@ -48,7 +48,7 @@ void VoiceSearchNavigationTabHelper::NavigationItemCommitted(
     const web::LoadCommittedDetails& load_details) {
   DCHECK_EQ(web_state_, web_state);
   if (will_navigate_to_voice_search_result_) {
-    load_details.item->SetUserData(
+    web_state->GetNavigationManager()->GetLastCommittedItem()->SetUserData(
         kNavigationMarkerKey, std::make_unique<VoiceSearchNavigationMarker>());
     will_navigate_to_voice_search_result_ = false;
   }