[ios] Add more checks for nil last committed item.

Catch more edge cases where last committed item can be nil if
navigating when about://newtab is still loading.

Bug: 931273
TBR: sdefresne@
Change-Id: I29bdd0a5f8c5b6e9a098face36c87c6bc0b5ee13
Reviewed-on: https://chromium-review.googlesource.com/c/1469427
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631915}

components/history/ios/browser/web_state_top_sites_observer.mm

@@ -40,7 +40,10 @@
 void WebStateTopSitesObserver::DidFinishNavigation(
     web::WebState* web_state,
     web::NavigationContext* navigation_context) {
-  if (top_sites_ && navigation_context->HasCommitted()) {
+  // TODO(crbug.com/931841): Remove GetLastCommittedItem nil check once
+  // HasComitted has been fixed.
+  if (top_sites_ && navigation_context->HasCommitted() &&
+      web_state->GetNavigationManager()->GetLastCommittedItem()) {
     top_sites_->OnNavigationCommitted(
         web_state->GetNavigationManager()->GetLastCommittedItem()->GetURL());
   }

ios/chrome/browser/history/history_tab_helper.mm

@@ -105,7 +105,10 @@
     return;
   }
 
-  if (!navigation_context->HasCommitted()) {
+  // TODO(crbug.com/931841): Remove GetLastCommittedItem nil check once
+  // HasComitted has been fixed.
+  if (!navigation_context->HasCommitted() ||
+      !web_state_->GetNavigationManager()->GetLastCommittedItem()) {
     // Navigation was replaced or aborted.
     return;
   }

ios/chrome/browser/infobars/infobar_manager_impl.mm

@@ -75,7 +75,10 @@
     web::WebState* web_state,
     web::NavigationContext* navigation_context) {
   DCHECK_EQ(web_state_, web_state);
-  if (navigation_context->HasCommitted()) {
+  // TODO(crbug.com/931841): Remove GetLastCommittedItem nil check once
+  // HasComitted has been fixed.
+  if (navigation_context->HasCommitted() &&
+      web_state->GetNavigationManager()->GetLastCommittedItem()) {
     OnNavigation(CreateNavigationDetails(
         web_state->GetNavigationManager()->GetLastCommittedItem(),
         navigation_context->IsSameDocument()));