[iOS][Getaway] Add dismissal fade animation.

Adds a fading alpha animation when dismissing the blocking UI.

Bug: none
Change-Id: I014b8e67d230045e130343894cb4759ce05b8ac5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593633
Auto-Submit: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837553}

ios/chrome/browser/ui/browser_view/browser_view_controller.mm

@@ -4712,8 +4712,15 @@ NSString* const kBrowserViewControllerSnackbarCategory =
 
     [self.view addSubview:self.blockingView];
     AddSameConstraints(self.view, self.blockingView);
+    self.blockingView.alpha = 1;
   } else {
-    [self.blockingView removeFromSuperview];
+    [UIView animateWithDuration:0.2
+        animations:^{
+          self.blockingView.alpha = 0;
+        }
+        completion:^(BOOL finished) {
+          [self.blockingView removeFromSuperview];
+        }];
   }
 }
 

ios/chrome/browser/ui/tab_switcher/tab_grid/grid/grid_view_controller.mm

@@ -563,9 +563,16 @@ NSIndexPath* CreateIndexPath(NSInteger index) {
     }
 
     [self.view addSubview:self.blockingView];
+    self.blockingView.alpha = 1;
     AddSameConstraints(self.collectionView.frameLayoutGuide, self.blockingView);
   } else {
-    [self.blockingView removeFromSuperview];
+    [UIView animateWithDuration:0.2
+        animations:^{
+          self.blockingView.alpha = 0;
+        }
+        completion:^(BOOL finished) {
+          [self.blockingView removeFromSuperview];
+        }];
   }
 }