[Start] Fix the closing tab animation in grid tab switcher surface.

When we switch to grid tab switcher surface from start surface,
#prepareTabSwitcherView won't be called since it's not a layout change.
So |mOriginalAnimator| is not initialized and closing tab animation is
not shown.
This CL adds a null check for |mOriginalAnimator| to fix this problem.

Before:
https://drive.google.com/file/d/1am7UfBY_7VY8BCuBStyI9sCxhfnvoKUA/view?usp=sharing&resourcekey=0-DuOCztQLuQPGjZw3n9aqSw
After:
https://drive.google.com/file/d/1PbiNL6OfBTxEBFhk246qeT1kglDpwzk_/view?usp=sharing&resourcekey=0-vcPbhEzFheDHy-JaAysfaQ

(cherry picked from commit deeb007)

Bug: 1200882
Change-Id: I6372b9dceeaaa5ec78cfef70cee0d972b13b5d2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3664819
Reviewed-by: Mei Liang <meiliang@chromium.org>
Commit-Queue: Hao Dong <spdonghao@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1007646}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3669600
Commit-Queue: Krishna Govind <govind@chromium.org>
Reviewed-by: Krishna Govind <govind@chromium.org>
Owners-Override: Krishna Govind <govind@chromium.org>
Cr-Commit-Position: refs/branch-heads/5060@{#289}
Cr-Branched-From: b83393d-refs/heads/main@{#1002911}

chrome/android/features/tab_ui/java/src/org/chromium/chrome/browser/tasks/tab_management/TabListRecyclerView.java

@@ -149,6 +149,8 @@ public void onAnimationEnd(Animator animation) {
     private ImageView mShadowImageView;
     private int mShadowTopOffset;
     private TabListOnScrollListener mScrollListener;
+    // It is null when gts-tab animation is disabled or switching from Start surface to GTS.
+    @Nullable
     private RecyclerView.ItemAnimator mOriginalAnimator;
 
     /**
@@ -203,7 +205,12 @@ public void onAnimationEnd(Animator animation) {
                 mFadeInAnimator = null;
                 mListener.finishedShowing();
                 // Restore the original value.
-                setItemAnimator(mOriginalAnimator);
+                // TODO(crbug.com/1315676): Remove the null check after decoupling Start surface
+                // layout and grid tab switcher layout.
+                if (mOriginalAnimator != null) {
+                    setItemAnimator(mOriginalAnimator);
+                    mOriginalAnimator = null;
+                }
                 setShadowVisibility(computeVerticalScrollOffset() > 0);
                 if (mDynamicView != null) {
                     mDynamicView.dropCachedBitmap();