[iOS][Thumb Strip] Remove retain cycle

Removes the following retain cycle:
self-> animator-> animations block -> self

Bug: N/A
Change-Id: I614f6d81f328eed8e0707f87fa2a9065d00089ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617863
Auto-Submit: Javier Flores <javierrobles@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#841985}

ios/chrome/browser/ui/gestures/view_revealing_vertical_pan_handler.mm

@@ -167,13 +167,14 @@ const CGFloat kAnimationDuration = 0.25f;
   }
   [self willAnimateViewReveal];
   [self.animator stopAnimation:YES];
+
+  __weak ViewRevealingVerticalPanHandler* weakSelf = self;
   self.animator = [[UIViewPropertyAnimator alloc]
       initWithDuration:kAnimationDuration
           dampingRatio:1
             animations:^() {
-              [self animateToNextViewRevealState];
+              [weakSelf animateToNextViewRevealState];
             }];
-  __weak ViewRevealingVerticalPanHandler* weakSelf = self;
   [self.animator addCompletion:^(UIViewAnimatingPosition finalPosition) {
     if (!weakSelf.animator.reversed) {
       weakSelf.currentState = weakSelf.nextState;