[ios] On closing last incognito, return to regular tabs

Users were feeling like they were "trapped in incognito". This solution
returns the user to the regular tabs tab switcher when the last
incognito tab is closed. This behavior is similar to pre-bijou.

Bug: 891396
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: Ifa9ba23bcc4d0f6029dbff73e60acd42993e1272
Reviewed-on: https://chromium-review.googlesource.com/c/1259481Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596523}

ios/chrome/browser/ui/tab_grid/tab_grid_view_controller.mm

@@ -1232,7 +1232,14 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
   [self configureButtonsForActiveAndCurrentPage];
   if (gridViewController == self.regularTabsViewController) {
     self.topToolbar.pageControl.regularTabCount = count;
+  } else if (gridViewController == self.incognitoTabsViewController) {
+    if (count == 0) {
+      [self.topToolbar.pageControl setSelectedPage:TabGridPageRegularTabs
+                                          animated:YES];
+      [self setCurrentPage:TabGridPageRegularTabs animated:YES];
+    }
   }
+
   [self broadcastIncognitoContentVisibility];
 }