[ios] Show the active webstate.

This CL updates the switch to the active webstate according to the
tapped tab in the TabStrip.
This CL doesn't update the tabstrip UI, i.e doesn't highlight the
selected tab.

Bug: 1128249
Change-Id: I89041c22d27df2d171821c6d5fc7342c99ab06db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550711Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Nazerke Kalidolda <nazerke@google.com>
Cr-Commit-Position: refs/heads/master@{#830132}

ios/chrome/browser/ui/tab_switcher/tab_strip/tab_strip_consumer_delegate.h

@@ -13,6 +13,9 @@
 // Tells the receiver to insert a new item in the tabstrip.
 - (void)addNewItem;
 
+// Tells the receiver to show to the selected tab.
+- (void)selectTab:(int)index;
+
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_TAB_SWITCHER_TAB_STRIP_TAB_STRIP_CONSUMER_DELEGATE_H_

ios/chrome/browser/ui/tab_switcher/tab_strip/tab_strip_mediator.mm

@@ -197,6 +197,13 @@ web::WebState* GetWebStateWithId(WebStateList* web_state_list,
       WebStateOpener());
 }
 
+- (void)selectTab:(int)index {
+  if (!self.webStateList)
+    return;
+
+  _webStateList->ActivateWebStateAt(index);
+}
+
 #pragma mark - Private
 
 // Calls |-populateItems:selectedItemID:| on the consumer.

ios/chrome/browser/ui/tab_switcher/tab_strip/tab_strip_view_controller.mm

@@ -178,4 +178,12 @@ const CGFloat kNewTabButtonBottomImageInset = -2.0;
   [self.delegate addNewItem];
 }
 
+#pragma mark - UICollectionViewDelegate
+
+- (void)collectionView:(UICollectionView*)collectionView
+    didSelectItemAtIndexPath:(NSIndexPath*)indexPath {
+  int index = indexPath.item;
+  [self.delegate selectTab:index];
+}
+
 @end