Add functionality to the New Tab button cell in the Grid View.

Open a new tab when the user taps the New Tab button cell.
Extend the GridViewControllerDelegate protocol to tell the delegate when
a new tab is opened.

Bug: 1127604, 1135329
Change-Id: If35f357df1c6a5f8eb9b181e76fb78ca241e0656
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2449994
Commit-Queue: Roberto Moura <mouraroberto@google.com>
Auto-Submit: Roberto Moura <mouraroberto@google.com>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815851}

ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.h

@@ -27,6 +27,10 @@
 // |gridViewController|.
 - (void)gridViewController:(GridViewController*)gridViewController
         didCloseItemWithID:(NSString*)itemID;
+// Tells the delegate that the plus sign was tapped in |gridViewController|,
+// i.e., there was an intention to create a new item.
+- (void)didTapPlusSignInGridViewController:
+    (GridViewController*)gridViewController;
 // Tells the delegate that the item at |sourceIndex| was moved to
 // |destinationIndex|.
 - (void)gridViewController:(GridViewController*)gridViewController

ios/chrome/browser/ui/tab_grid/grid/grid_view_controller_unittest.mm

@@ -51,6 +51,11 @@
   // No-op for unittests. This is only called when a user taps to close a cell,
   // not generically when items are removed from the data source.
 }
+- (void)didTapPlusSignInGridViewController:
+    (GridViewController*)gridViewController {
+  // No-op for unittests. This is only called when a user taps on a
+  // plus sign cell, not generically when items are added to the data source.
+}
 @end
 
 class GridViewControllerTest : public RootViewControllerTest {

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

@@ -1191,6 +1191,17 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
   }
 }
 
+- (void)didTapPlusSignInGridViewController:
+    (GridViewController*)gridViewController {
+  if (gridViewController == self.regularTabsViewController) {
+    [self.regularTabsDelegate addNewItem];
+    // TODO(crbug.com/1135329): Record when a new regular tab is opened.
+  } else if (gridViewController == self.incognitoTabsViewController) {
+    [self.incognitoTabsDelegate addNewItem];
+    // TODO(crbug.com/1135329): Record when a new incognito tab is opened.
+  }
+}
+
 - (void)gridViewController:(GridViewController*)gridViewController
          didMoveItemWithID:(NSString*)itemID
                    toIndex:(NSUInteger)destinationIndex {