[ARC] Converts part of ios/web/ui to ARC.

ios/web/web_state/ui/crw_touch_tracking_recognizer.h

 // Copyright 2012 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_WEB_WEB_STATE_UI_CRW_TOUCH_TRACKING_RECOGNIZER_H_
 #define IOS_WEB_WEB_STATE_UI_CRW_TOUCH_TRACKING_RECOGNIZER_H_
 
 #import <UIKit/UIGestureRecognizerSubclass.h>
 
 // Methods implemented by the delegate of the CRWTouchTrackingRecognizer.
 @protocol CRWTouchTrackingDelegate
 
 // Called with YES when touches began, with NO when touches are ended or
 // cancelled.
 - (void)touched:(BOOL)touched;
 
 @end
 
 // UIGestureRecognizer subclass that informs delegate about touches using
 // simplified interface.
 @interface CRWTouchTrackingRecognizer : UIGestureRecognizer
 
 // CRWTouchTrackingRecognizer delegate.
-@property(nonatomic, assign) id<CRWTouchTrackingDelegate> touchTrackingDelegate;
+@property(nonatomic, weak) id<CRWTouchTrackingDelegate> touchTrackingDelegate;
 
 // Designated initializer for CRWTouchTrackingRecognizer.
 - (id)initWithDelegate:(id<CRWTouchTrackingDelegate>)delegate;
 
 @end
 
 #endif  // IOS_WEB_WEB_STATE_UI_CRW_TOUCH_TRACKING_RECOGNIZER_H_