[ObjC ARC] Converts ios/chrome/browser/ui/tab_switcher:tab_switcher to ARC.

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

Index: ios/chrome/browser/ui/tab_switcher/tab_switcher_tab_strip_placeholder_view.mm
diff --git a/ios/chrome/browser/ui/tab_switcher/tab_switcher_tab_strip_placeholder_view.mm b/ios/chrome/browser/ui/tab_switcher/tab_switcher_tab_strip_placeholder_view.mm
index fda92e3d8ea2271b2b2eb0071f57700c6a8da20b..90255dff40bbe82765a8a34cd53d69b20fafa80f 100644
--- a/ios/chrome/browser/ui/tab_switcher/tab_switcher_tab_strip_placeholder_view.mm
+++ b/ios/chrome/browser/ui/tab_switcher/tab_switcher_tab_strip_placeholder_view.mm
@@ -8,11 +8,13 @@
 
 #import <QuartzCore/QuartzCore.h>
 
-#import "base/ios/weak_nsobject.h"
 #include "base/mac/foundation_util.h"
-#include "base/mac/scoped_nsobject.h"
 #include "ios/chrome/browser/ui/rtl_geometry.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 // Returns the animation drag coefficient set by the iPhone simulator.
 // This is useful when debugging with the simulator because when
 // "slow animation" mode is toggled, it only impacts UIKit animations not
@@ -106,16 +108,15 @@ const CGFloat kTabFoldAnimationDuration = 0.15;
   _animatingFold = YES;
   [self setUserInteractionEnabled:NO];
   [CATransaction begin];
-  base::WeakNSObject<TabSwitcherTabStripPlaceholderView> weakSelf(self);
+  __weak TabSwitcherTabStripPlaceholderView* weakSelf = self;
   [CATransaction setCompletionBlock:^{
-    base::scoped_nsobject<TabSwitcherTabStripPlaceholderView> strongSelf(
-        [weakSelf retain]);
+    TabSwitcherTabStripPlaceholderView* strongSelf = weakSelf;
     if (!strongSelf) {
       if (completion)
         completion();
       return;
     }
-    strongSelf.get()->_animatingFold = NO;
+    strongSelf->_animatingFold = NO;
     [strongSelf setUserInteractionEnabled:YES];
     if (completion)
       completion();