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..335af5512ef96bdc3e7a1e54b526cecba13ffcf0 100644
--- a/ios/web/navigation/navigation_item_impl.mm
+++ b/ios/web/navigation/navigation_item_impl.mm
@@ -204,7 +204,7 @@ bool NavigationItemImpl::HasPostData() const {
 }
 
 NSDictionary* NavigationItemImpl::GetHttpRequestHeaders() const {
-  return [[http_request_headers_ copy] autorelease];
+  return [http_request_headers_ copy];

[Eugene But (OOO till 7-30), 2016/08/01 21:13:48]

This looks like a leak because compiler does not know that autorelease is needed
here.

[stkhapugin, 2016/08/18 17:09:36]

Autorelease will be inserted by clang with ARC.

 }
 
 void NavigationItemImpl::AddHttpRequestHeaders(
@@ -220,7 +220,7 @@ 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);
 }
 
 NSString* NavigationItemImpl::GetSerializedStateObject() const {
@@ -244,7 +244,7 @@ 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 {