[ios] Check for non nil webState, not tab, for NewTabPageTabHelper.

Bug: 934294
Change-Id: I34eac982df5e4ce23a7e748debe9b118cd88afb4
Reviewed-on: https://chromium-review.googlesource.com/c/1481491
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634365}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -2813,7 +2813,7 @@ NSString* const kBrowserViewControllerSnackbarCategory =
 
 - (id)nativeControllerForTab:(Tab*)tab {
   id nativeController = tab.webController.nativeController;
-  if (tab) {
+  if (tab.webState) {
     NewTabPageTabHelper* NTPHelper =
         NewTabPageTabHelper::FromWebState(tab.webState);
     if (NTPHelper && NTPHelper->IsActive())

ios/chrome/browser/ui/side_swipe/side_swipe_controller.mm

@@ -504,7 +504,6 @@ const NSUInteger kIpadGreySwipeTabCount = 8;
 - (void)handleiPhoneTabSwipe:(SideSwipeGestureRecognizer*)gesture {
   if (gesture.state == UIGestureRecognizerStateBegan) {
     Tab* currentTab = [model_ currentTab];
-    DCHECK(currentTab.webState);
 
     inSwipe_ = YES;
 
@@ -513,10 +512,13 @@ const NSUInteger kIpadGreySwipeTabCount = 8;
     // Add horizontal stack view controller.
     // TODO(crbug.com/904992): Do not use SnapshotGeneratorDelegate from
     // SideSwipeController.
-    CGFloat headerHeight =
-        [self.snapshotDelegate snapshotGenerator:nil
-                   snapshotEdgeInsetsForWebState:currentTab.webState]
-            .top;
+    CGFloat headerHeight = 0;
+    if (currentTab.webState) {
+      headerHeight =
+          [self.snapshotDelegate snapshotGenerator:nil
+                     snapshotEdgeInsetsForWebState:currentTab.webState]
+              .top;
+    }
 
     if (tabSideSwipeView_) {
       [tabSideSwipeView_ setFrame:frame];
@@ -537,8 +539,10 @@ const NSUInteger kIpadGreySwipeTabCount = 8;
     }
 
     // Ensure that there's an up-to-date snapshot of the current tab.
-    SnapshotTabHelper::FromWebState(currentTab.webState)
-        ->UpdateSnapshotWithCallback(nil);
+    if (currentTab.webState) {
+      SnapshotTabHelper::FromWebState(currentTab.webState)
+          ->UpdateSnapshotWithCallback(nil);
+    }
 
     // Layout tabs with new snapshots in the current orientation.
     [tabSideSwipeView_ updateViewsForDirection:gesture.direction];