[ios] Use WebStateList to back the tab grid.

ios/showcase/tab_grid/sc_tab_grid_coordinator.mm

Index: ios/showcase/tab_grid/sc_tab_grid_coordinator.mm
diff --git a/ios/showcase/tab_grid/sc_tab_grid_coordinator.mm b/ios/showcase/tab_grid/sc_tab_grid_coordinator.mm
index e47bd47bd143886f99dfe3e3853363aad5d3cc85..d9d2e141f4bdaba90221585f6d26d7e8905b59b4 100644
--- a/ios/showcase/tab_grid/sc_tab_grid_coordinator.mm
+++ b/ios/showcase/tab_grid/sc_tab_grid_coordinator.mm
@@ -40,16 +40,16 @@
 
 #pragma mark - TabGridDataSource
 
-- (NSUInteger)numberOfTabsInTabGrid {
+- (int)numberOfTabsInTabGrid {
   return 3;
 }
 
-- (NSString*)titleAtIndex:(NSInteger)index {
-  return [NSString stringWithFormat:@"Tab %" PRIdNS, index];
+- (NSString*)titleAtIndex:(int)index {
+  return [NSString stringWithFormat:@"Tab %d", index];
 }
 
-- (NSInteger)indexOfActiveTab {
-  return NSNotFound;
+- (int)indexOfActiveTab {
+  return kTabGridDataSourceInvalidIndex;
 }
 
 @end