[ios] darken TableViewPresentationController modal shield to 0.4 alpha

Before: https://drive.google.com/file/d/1lkH7kzBhoc02yFuqiJFkArO5XGoRkgv4/view?usp=sharing
After: https://drive.google.com/file/d/1HVNhe0k4rWch1oO6TFFLQkU17SL9fqLg/view?usp=sharing

Bug: 856626
Change-Id: I022b5d8b54828855f836e0428d29d0ffde6bcdfd
Reviewed-on: https://chromium-review.googlesource.com/1117743Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570967}

ios/chrome/browser/ui/table_view/table_view_presentation_controller.mm

@@ -215,7 +215,7 @@ const CGFloat kTableViewMaxWidth = 414.0;
 // if it is called from within an animation block.
 - (void)updateDimmingShieldForModal:(BOOL)modal {
   if (modal) {
-    self.dimmingShield.backgroundColor = [UIColor colorWithWhite:0.0 alpha:0.3];
+    self.dimmingShield.backgroundColor = [UIColor colorWithWhite:0.0 alpha:0.4];
     self.shadowImage.alpha = 0.0;
   } else {
     self.dimmingShield.backgroundColor = [UIColor clearColor];