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

ios/web/web_state/ui/crw_web_view_content_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.
 
 #import "ios/web/public/web_state/ui/crw_web_view_content_view.h"
 
 #import <WebKit/WebKit.h>
 
 #include "base/logging.h"
 #include "base/mac/scoped_nsobject.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 // Background color RGB values for the content view which is displayed when the
 // |_webView| is offset from the screen due to user interaction. Displaying this
 // background color is handled by UIWebView but not WKWebView, so it needs to be
 // set in CRWWebViewContentView to support both. The color value matches that
 // used by UIWebView.
 const CGFloat kBackgroundRGBComponents[] = {0.75f, 0.74f, 0.76f};
 
 }  // namespace
@@ skipping 17 matching lines @@
 
 @synthesize shouldUseInsetForTopPadding = _shouldUseInsetForTopPadding;
 
 - (instancetype)initWithWebView:(UIView*)webView
                      scrollView:(UIScrollView*)scrollView {
   self = [super initWithFrame:CGRectZero];
   if (self) {
     DCHECK(webView);
     DCHECK(scrollView);
     DCHECK([scrollView isDescendantOfView:webView]);
-    _webView.reset([webView retain]);
-    _scrollView.reset([scrollView retain]);
+    _webView.reset(webView);
+    _scrollView.reset(scrollView);
   }
   return self;
 }
 
 - (instancetype)initForTesting {
   return [super initWithFrame:CGRectZero];
 }
 
 - (instancetype)initWithCoder:(NSCoder*)decoder {
   NOTREACHED();
@@ skipping 104 matching lines @@
 
 - (void)updateWebViewFrame {
   CGRect webViewFrame = self.bounds;
   webViewFrame.size.height -= _topContentPadding;
   webViewFrame.origin.y += _topContentPadding;
 
   self.webView.frame = webViewFrame;
 }
 
 @end