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

ios/web/web_state/ui/crw_web_view_content_view.mm

Index: ios/web/web_state/ui/crw_web_view_content_view.mm
diff --git a/ios/web/web_state/ui/crw_web_view_content_view.mm b/ios/web/web_state/ui/crw_web_view_content_view.mm
index df2962af45cf8c70173413386bf89e653d0b8be1..561f11cce7ea9b39595e813a7dffbfc93b1ce368 100644
--- a/ios/web/web_state/ui/crw_web_view_content_view.mm
+++ b/ios/web/web_state/ui/crw_web_view_content_view.mm
@@ -9,6 +9,10 @@
 #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
@@ -46,8 +50,8 @@ const CGFloat kBackgroundRGBComponents[] = {0.75f, 0.74f, 0.76f};
     DCHECK(webView);
     DCHECK(scrollView);
     DCHECK([scrollView isDescendantOfView:webView]);
-    _webView.reset([webView retain]);
-    _scrollView.reset([scrollView retain]);
+    _webView.reset(webView);
+    _scrollView.reset(scrollView);
   }
   return self;
 }