[ios] Update TabGrid's activePage when closing last incognito tab

When closing the last incognito tab, the incognito page is no longer
the active page. It should be updated.

Bug: 1011054
Change-Id: If2df5c8398bbc13ea533652296dbf124a63d650c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872158
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707896}

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

@@ -1118,6 +1118,7 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
     // the incognito tabs.
     if (count == 0 && self.currentPage == TabGridPageIncognitoTabs) {
       // Show the regular tabs to the user if the last incognito tab is closed.
+      self.activePage = TabGridPageRegularTabs;
       if (self.viewLoaded && self.view.window) {
         // Visibly scroll to the regular tabs panel after a slight delay when
         // the user is already in the tab switcher.