[ios] Snapshots enabled in tab grid

Bug: 804529
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I5a410831b3d6c7dabe066330b1250761a9b21243
Reviewed-on: https://chromium-review.googlesource.com/944060
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@{#540378}

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

@@ -100,6 +100,9 @@
   self.incognitoTabsMediator = [[TabGridMediator alloc]
       initWithConsumer:mainViewController.incognitoTabsConsumer];
   self.incognitoTabsMediator.tabModel = self.incognitoTabModel;
+
+  mainViewController.regularTabsImageDataSource = self.regularTabsMediator;
+  mainViewController.incognitoTabsImageDataSource = self.incognitoTabsMediator;
 }
 
 - (void)stop {