[iOS] Animate tab grid cell deletion.

This CL customizes the deletion animation for tab grid cells via the
standard UICollectionViewLayout APIs.

Animation is done by adjusting the final layout attributes of the cell
in -finalLayoutAttributesForDisappearingItemAtIndexPath:. Because cells
may disappear for non-deleting reasons, the layout also needs to track
which cells are being deleted during each update cycle.

Bug: 804553
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I8566a5aff832ac8844d2d89639599b1b2372a27d
Reviewed-on: https://chromium-review.googlesource.com/964223Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543389}

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

@@ -15,11 +15,13 @@ const CGFloat kInterTabSpacing = 20.0f;
 @interface GridLayout ()
 @property(nonatomic, assign) CGFloat startingTabWidth;
 @property(nonatomic, assign) CGFloat maxTabWidth;
+@property(nonatomic, strong) NSArray<NSIndexPath*>* indexPathsOfDeletingItems;
 @end
 
 @implementation GridLayout
 @synthesize startingTabWidth = _startingTabWidth;
 @synthesize maxTabWidth = _maxTabWidth;
+@synthesize indexPathsOfDeletingItems = _indexPathsOfDeletingItems;
 
 - (instancetype)init {
   if (self = [super init]) {
@@ -42,6 +44,44 @@ const CGFloat kInterTabSpacing = 20.0f;
   }
 }
 
+- (void)prepareForCollectionViewUpdates:
+    (NSArray<UICollectionViewUpdateItem*>*)updateItems {
+  NSMutableArray<NSIndexPath*>* deletingItems =
+      [NSMutableArray arrayWithCapacity:updateItems.count];
+  for (UICollectionViewUpdateItem* item in updateItems) {
+    if (item.updateAction == UICollectionUpdateActionDelete) {
+      [deletingItems addObject:item.indexPathBeforeUpdate];
+    }
+  }
+  self.indexPathsOfDeletingItems = [deletingItems copy];
+}
+
+- (UICollectionViewLayoutAttributes*)
+finalLayoutAttributesForDisappearingItemAtIndexPath:
+    (NSIndexPath*)itemIndexPath {
+  UICollectionViewLayoutAttributes* attributes =
+      [super finalLayoutAttributesForDisappearingItemAtIndexPath:itemIndexPath];
+  // Disappearing items that aren't being deleted just use the default
+  // attributes.
+  if (![self.indexPathsOfDeletingItems containsObject:itemIndexPath]) {
+    return attributes;
+  }
+  // Cells being deleted fade out, are scaled down, and drop downwards slightly.
+  attributes.alpha = 0.0;
+  // Scaled down to 60%.
+  CGAffineTransform transform =
+      CGAffineTransformScale(attributes.transform, 0.6, 0.6);
+  // Translated down (positive-y direction) by 50% of the cell cell size.
+  transform =
+      CGAffineTransformTranslate(transform, 0, attributes.size.height * 0.5);
+  attributes.transform = transform;
+  return attributes;
+}
+
+- (void)finalizeCollectionViewUpdates {
+  self.indexPathsOfDeletingItems = @[];
+}
+
 #pragma mark - Private
 
 // This sets the appropriate itemSize given the width of the collection view.