Update viewForTab to use BVC own tabmodel

Previously it used to use tab.tabmodel which is going to be deleted.

Bug: 899839
Change-Id: I86584dfcd6bae44b289585fe1562c2f160715406
Reviewed-on: https://chromium-review.googlesource.com/c/1321669
Commit-Queue: Mohammad Refaat <mrefaat@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611294}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -2644,9 +2644,10 @@ applicationCommandEndpoint:(id<ApplicationCommands>)applicationCommandEndpoint
   if (NTPHelper && NTPHelper->IsActive()) {
     return _ntpCoordinatorsForWebStates[webState].viewController.view;
   }
+  DCHECK([self.tabModel indexOfTab:tab] != NSNotFound);
   // TODO(crbug.com/904588): Move |RecordPageLoadStart| to TabUsageRecorder.
-  if (webState->IsEvicted() && [tab.parentTabModel tabUsageRecorder]) {
-    [tab.parentTabModel tabUsageRecorder]->RecordPageLoadStart(webState);
+  if (webState->IsEvicted() && [self.tabModel tabUsageRecorder]) {
+    [self.tabModel tabUsageRecorder] -> RecordPageLoadStart(webState);
   }
   if (!webState->IsCrashed()) {
     // Load the page if it was evicted by browsing data clearing logic.