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

ios/web/web_state/page_viewport_state.mm

 // Copyright 2016 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/web_state/page_viewport_state.h"
 
 #include "base/logging.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 // Parses a double from |viewport_content|.  |viewport_content| is expected to
 // have no leading whitespace.
 double ParseDouble(NSString* viewport_content) {
   double value = [viewport_content doubleValue];
   // |-doubleValue| returns zero when parsing a non-numerical string, so check
   // that the string actually contains zero to verify that the value was
   // represented in the string.
   if (!value && ![viewport_content hasPrefix:@"0"])
     value = NAN;
@@ skipping 56 matching lines @@
         minimum_zoom_scale_ = ParseDouble(value);
       } else if (![name caseInsensitiveCompare:@"maximum-scale"]) {
         maximum_zoom_scale_ = ParseDouble(value);
       } else if (![name caseInsensitiveCompare:@"initial-scale"]) {
         initial_zoom_scale_ = ParseDouble(value);
       }
     }
   }
 }
 
-}  // namespace web
+}  // namespace web