Add last committed item URL validity check in SearchEngineTabHelper.

This CL adds a validity check for FaviconDriver::GetActiveURL().
GetActiveURL() depends on GetLastCommittedItem, and can be nil at various
times -- such as during session restore on startup.

Bug: 908240
Change-Id: I32bcf8e93b862fd87ae22bce6c59cbb9c4ba2f5d
Reviewed-on: https://chromium-review.googlesource.com/c/1350873Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612263}

ios/chrome/browser/search_engines/search_engine_tab_helper.mm

@@ -105,8 +105,9 @@ void SearchEngineTabHelper::OnFaviconUpdated(
       ios::ChromeBrowserState::FromBrowserState(web_state_->GetBrowserState());
   TemplateURLService* url_service =
       ios::TemplateURLServiceFactory::GetForBrowserState(browser_state);
-  if (url_service && url_service->loaded())
-    url_service->UpdateProviderFavicons(driver->GetActiveURL(), icon_url);
+  const GURL potential_search_url = driver->GetActiveURL();
+  if (url_service && url_service->loaded() && potential_search_url.is_valid())
+    url_service->UpdateProviderFavicons(potential_search_url, icon_url);
 }
 
 // When the page is loaded, checks if |searchable_url_| has a value generated