[iOS] Fix constness issue in side_swipe_gesture_recognizer

ios/chrome/browser/ui/side_swipe_gesture_recognizer.mm

 // 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.
 
 #import "ios/chrome/browser/ui/side_swipe_gesture_recognizer.h"
 
 #include <cmath>
 
 #include "base/logging.h"
 
 namespace {
 
 // The absolute maximum swipe angle from |x = y| for a swipe to begin.
-CGFloat kMaxSwipeYAngle = 65;
+const CGFloat kMaxSwipeYAngle = 65;
 // The distance between touches for a swipe to begin.
-CGFloat kMinSwipeXThreshold = 4;
+const CGFloat kMinSwipeXThreshold = 4;
 
 }  // namespace
 
 @implementation SideSwipeGestureRecognizer {
   // Starting point of swipe.
   CGPoint _startPoint;
   // Expected direction of the swipe, based on starting point.
   UISwipeGestureRecognizerDirection _direction;
 }
 
@@ skipping 70 matching lines @@
   _startPoint = CGPointZero;
   [super touchesEnded:touches withEvent:event];
 }
 
 - (void)touchesCancelled:(NSSet*)touches withEvent:(UIEvent*)event {
   _startPoint = CGPointZero;
   [super touchesCancelled:touches withEvent:event];
 }
 
 @end