Release TabBarTransitionContext when no longer required

ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.mm

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/tab_switcher/tab_switcher_controller.h"
 
 #include "base/ios/block_types.h"
 #include "base/ios/weak_nsobject.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/metrics/user_metrics.h"
@@ skipping 664 matching lines @@
   [toolbarShadowImageView setImage:NativeImage(IDR_IOS_TOOLBAR_SHADOW)];
   [placeholderView addSubview:toolbarShadowImageView];
 
   [self.view addSubview:placeholderView];
 
   [selectedCell setHidden:YES];
   toolbarScreenshotImageView.alpha =
       (transitionType == TransitionType::TRANSITION_DISMISS) ? 0 : 1.0;
 
   base::WeakNSObject<TabSwitcherController> weakSelf(self);
-  void (^completionBlock)(BOOL) = ^(BOOL) {
+  void (^completionBlock)(BOOL) = ^(BOOL finished) {
     base::scoped_nsobject<TabSwitcherController> strongSelf([weakSelf retain]);
 
     [tabStripPlaceholderView removeFromSuperview];
     [toolbarScreenshotImageView removeFromSuperview];
     [selectedCell setHidden:NO];
     [placeholderView removeFromSuperview];
 
-    if (transitionType == TransitionType::TRANSITION_DISMISS)
+    if (transitionType == TransitionType::TRANSITION_DISMISS) {
       [strongSelf restoreWindowBackgroundColor];
+      if (finished) {
+        [strongSelf setTransitionContext:nil];
+      }
+    }
     [[[strongSelf delegate] tabSwitcherTransitionToolbarOwner]
         reparentToolbarController];
     [[strongSelf view] setUserInteractionEnabled:YES];
     completion();
   };
 
   ProceduralBlock animationBlock = ^{
     toolbarScreenshotImageView.alpha =
         transitionType == TransitionType::TRANSITION_DISMISS ? 1.0 : 0;
     tabStripPlaceholderView.frame = tabStripFinalFrame;
@@ skipping 531 matching lines @@
         base::UserMetricsAction("MobileTabSwitcherCloseNonIncognitoTab"));
   }
 }
 
 - (void)tabSwitcherPanelControllerDidUpdateOverlayViewVisibility:
     (TabSwitcherPanelController*)tabSwitcherPanelController {
   [_tabSwitcherView updateOverlayButtonState];
 }
 
 @end