Reland of [ObjC ARC] Converts ios/chrome/browser/ui/contextual_search:contextual_search to ARC.

ios/chrome/browser/ui/contextual_search/contextual_search_mask_view.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.
 
 #include "ios/chrome/browser/ui/contextual_search/contextual_search_mask_view.h"
 
 #include "base/logging.h"
 #import "ios/chrome/browser/ui/contextual_search/contextual_search_panel_view.h"
 #import "ios/chrome/browser/ui/uikit_ui_util.h"
 
 // Linearly map |delta| in the range [0, 1] to a value in [min, max].
 #define LERP(min, max, delta) (min * (1 - delta) + max * delta)
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 const CGFloat kPhoneMaskLimit = 1.0;
 const CGFloat kPadMaskLimit = 0.8;
 }
 
 @implementation ContextualSearchMaskView {
   CGFloat _maskLimit;
 }
 
 - (instancetype)init {
@@ skipping 45 matching lines @@
   ratio = LERP(0, _maskLimit, ratio);
 
   self.alpha = ratio * ratio;
 }
 
 - (void)panelWillPromote:(ContextualSearchPanelView*)panel {
   [panel removeMotionObserver:self];
 }
 
 @end