Fix WebFaviconDriver::GetActiveURL.

According to the code's comments (and caller's expectations), the method
should return last committed URL instead of visible URL.

"last committed URL" is URL of the page, while "visible URL" is the URL
which should be displayed in the omnibox (one that the user has typed
for example).

Bug: 897506
Change-Id: I13148cc2dee62c707cbceebab0d8d19622f942b8
Reviewed-on: https://chromium-review.googlesource.com/c/1316427Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605363}

components/favicon/ios/web_favicon_driver.mm

@@ -60,7 +60,7 @@ bool WebFaviconDriver::FaviconIsValid() const {
 
 GURL WebFaviconDriver::GetActiveURL() {
   web::NavigationItem* item =
-      web_state_->GetNavigationManager()->GetVisibleItem();
+      web_state_->GetNavigationManager()->GetLastCommittedItem();
   return item ? item->GetURL() : GURL();
 }