Add drop shadow to tab grid's thumbnails.

The drop shadows helps contrast thumbnails when they overlap one
another, i.e. during transitions between thumb strip and tab grid mode.

Bug: 1127604
Change-Id: I23b55172d112461ed63e6087203dbf780dcd1728
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487126
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Roberto Moura <mouraroberto@google.com>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819428}

ios/chrome/browser/ui/tab_grid/grid/grid_cell.mm

@@ -95,6 +95,12 @@ void PositionView(UIView* view, CGPoint point) {
     _snapshotView = snapshotView;
     _closeTapTargetButton = closeTapTargetButton;
 
+    self.layer.shadowColor = [UIColor blackColor].CGColor;
+    self.layer.shadowOffset = CGSizeMake(0, 0);
+    self.layer.shadowRadius = 4.0f;
+    self.layer.shadowOpacity = 0.5f;
+    self.layer.masksToBounds = NO;
+
     NSArray* constraints = @[
       [topBar.topAnchor constraintEqualToAnchor:contentView.topAnchor],
       [topBar.leadingAnchor constraintEqualToAnchor:contentView.leadingAnchor],