Make omnibox shortcuts background transparent.

Changes the shortcuts background color to clear.

Change-Id: I72d6ca59600fa29e191cf85144526aa573f828e9
Reviewed-on: https://chromium-review.googlesource.com/c/1296252
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603078}

ios/chrome/browser/ui/omnibox/popup/omnibox_popup_view_controller.mm

@@ -245,6 +245,7 @@ UIColor* BackgroundColorIncognito() {
   DCHECK(self.shortcutsViewController);
 
   UITableViewCell* cell = [[UITableViewCell alloc] init];
+  cell.backgroundColor = [UIColor clearColor];
   [self.shortcutsViewController willMoveToParentViewController:self];
   [self addChildViewController:self.shortcutsViewController];
   [cell.contentView addSubview:self.shortcutsViewController.view];