Converts parts of ios/web to ARC

ios/web/navigation/navigation_item_impl.mm

Index: ios/web/navigation/navigation_item_impl.mm
diff --git a/ios/web/navigation/navigation_item_impl.mm b/ios/web/navigation/navigation_item_impl.mm
index 0e979bc48f3a78ca095712b151f475c767d6c7e2..2cc32f570eb39b67e13c8a34ac418e95a7bb607a 100644
--- a/ios/web/navigation/navigation_item_impl.mm
+++ b/ios/web/navigation/navigation_item_impl.mm
@@ -14,6 +14,10 @@
 #include "ui/base/page_transition_types.h"
 #include "ui/gfx/text_elider.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 // Returns a new unique ID for use in NavigationItem during construction.  The
@@ -204,7 +208,7 @@ bool NavigationItemImpl::HasPostData() const {
 }
 
 NSDictionary* NavigationItemImpl::GetHttpRequestHeaders() const {
-  return [[http_request_headers_ copy] autorelease];
+  return [http_request_headers_ copy];
 }
 
 void NavigationItemImpl::AddHttpRequestHeaders(
@@ -220,7 +224,7 @@ void NavigationItemImpl::AddHttpRequestHeaders(
 
 void NavigationItemImpl::SetSerializedStateObject(
     NSString* serialized_state_object) {
-  serialized_state_object_.reset([serialized_state_object retain]);
+  serialized_state_object_.reset(serialized_state_object);

[kkhorimoto, 2016/08/19 23:15:43]

How does ARC function for C++ classes that have NSObject ivars?  Is it just
assumed that the variables use strong references?  If so, this is another place
where it'd be better to remove scoped_nsobjects.

[stkhapugin, 2016/08/22 11:37:00]

ARC treats all Cocoa object type variables in Objective-C++ files with ARC the
same. See other comment for why this is not going to be fixed now. 

 }
 
 NSString* NavigationItemImpl::GetSerializedStateObject() const {
@@ -244,7 +248,7 @@ bool NavigationItemImpl::ShouldSkipResubmitDataConfirmation() const {
 }
 
 void NavigationItemImpl::SetPostData(NSData* post_data) {
-  post_data_.reset([post_data retain]);
+  post_data_.reset(post_data);
 }
 
 NSData* NavigationItemImpl::GetPostData() const {