net: calculate the start of question section through the size of dns …

…Header type

Employ the same trick used at other places in DnsQuery to calculate the
beginning of the question section.

This allow us to write the code without the reinterpret_cast.

BUG=None
TEST=net_unittests --gtest_filter=*Dns*
R=ttuttle@chromium.org

Review URL: https://codereview.chromium.org/1579413002

Cr-Commit-Position: refs/heads/master@{#369433}

net/dns/dns_query.cc

@@ -27,8 +27,8 @@ DnsQuery::DnsQuery(uint16_t id, const base::StringPiece& qname, uint16_t qtype)
   header()->qdcount = base::HostToNet16(1);
 
   // Write question section after the header.
-  base::BigEndianWriter writer(reinterpret_cast<char*>(header() + 1),
-                               question_size());
+  base::BigEndianWriter writer(
+      io_buffer_->data() + sizeof(dns_protocol::Header), question_size());
   writer.WriteBytes(qname.data(), qname.size());
   writer.WriteU16(qtype);
   writer.WriteU16(dns_protocol::kClassIN);