[ios] Update tab grid background color

Bug: 849667
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I37bbca1646f3de0534331903b555417ef75fd448
Reviewed-on: https://chromium-review.googlesource.com/1090570Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565293}

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

@@ -119,6 +119,7 @@ NSUInteger GetPageIndexFromPage(TabGridPage page) {
 
 - (void)viewDidLoad {
   [super viewDidLoad];
+  self.view.backgroundColor = UIColorFromRGB(kGridBackgroundColor);
   [self setupScrollView];
   [self setupIncognitoTabsViewController];
   [self setupRegularTabsViewController];