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

ios/chrome/browser/ui/tab_switcher/tab_switcher_cache.h

 // 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.
 
 #ifndef IOS_CHROME_BROWSER_UI_TAB_SWITCHER_TAB_SWITCHER_CACHE_H_
 #define IOS_CHROME_BROWSER_UI_TAB_SWITCHER_TAB_SWITCHER_CACHE_H_
 
 #import <UIKit/UIKit.h>
 
 #import "ios/chrome/browser/tabs/tab_model_observer.h"
@@ skipping 11 matching lines @@
   void clear() {
     requestId = 0;
     sessionId = 0;
   }
 };
 
 // A class that provides access to resized snapshots of tabs. The snapshots are
 // cached and freed under memory pressure.
 @interface TabSwitcherCache : NSObject<TabModelObserver>
 
-@property(nonatomic, readonly) TabModel* mainTabModel;
+@property(weak, nonatomic, readonly) TabModel* mainTabModel;
 
 // Request a snapshot for the given |tab| of the given |size|, |completionBlock|
 // will be called with the requested snapshot. Must be called from the main
 // thread with a non nil |tab|, a non nil |completionBlock|, and a non
 // CGSizeZero |size|.
 // If the resized snapshot is cached, |completionBlock| is called synchronously.
 // Otherwise it is called asynchronously on the main thread. The block's
 // execution can be cancelled with |cancelPendingSnapshotRequest:|.
 // Requesting a snapshot for a given tab will cancel any previous request for
 // that same tab. The completion handler for a cancelled request may never be
@@ skipping 12 matching lines @@
 // request matches |pendingRequest|.
 - (void)cancelPendingSnapshotRequest:(PendingSnapshotRequest)pendingRequest;
 
 // Sets the main and incognito tab models.
 - (void)setMainTabModel:(TabModel*)mainTabModel
             otrTabModel:(TabModel*)otrTabModel;
 
 @end
 
 #endif  // IOS_CHROME_BROWSER_UI_TAB_SWITCHER_TAB_SWITCHER_CACHE_H_