factor out path id + packet number packing code from the QUIC aead code

net/quic/crypto/aead_base_decrypter_openssl.cc

Index: net/quic/crypto/aead_base_decrypter_openssl.cc
diff --git a/net/quic/crypto/aead_base_decrypter_openssl.cc b/net/quic/crypto/aead_base_decrypter_openssl.cc
index 635a1705288bfcd1fca317794b5d68d2ad267291..512b80471db706e7f3bc427809a57c948915251a 100644
--- a/net/quic/crypto/aead_base_decrypter_openssl.cc
+++ b/net/quic/crypto/aead_base_decrypter_openssl.cc
@@ -9,6 +9,7 @@
 
 #include "base/memory/scoped_ptr.h"
 #include "net/quic/quic_flags.h"
+#include "net/quic/quic_utils.h"
 
 using base::StringPiece;
 
@@ -93,15 +94,8 @@ bool AeadBaseDecrypter::DecryptPacket(QuicPathId path_id,
   const size_t nonce_size = nonce_prefix_size_ + sizeof(packet_number);
   memcpy(nonce, nonce_prefix_, nonce_prefix_size_);
   if (FLAGS_quic_include_path_id_in_iv) {
-    // Setting the nonce below relies on QuicPathId and QuicPacketNumber being
-    // specific sizes.
-    static_assert(sizeof(path_id) == 1, "Size of QuicPathId changed.");
-    static_assert(sizeof(packet_number) == 8,
-                  "Size of QuicPacketNumber changed.");
-    // Use path_id and lower 7 bytes of packet_number as lower 8 bytes of nonce.
     uint64_t path_id_packet_number =
-        (static_cast<uint64_t>(path_id) << 56) | packet_number;
-    DCHECK(path_id != kDefaultPathId || path_id_packet_number == packet_number);
+        QuicUtils::PackPathIdAndPacketNumber(path_id, packet_number);
     memcpy(nonce + nonce_prefix_size_, &path_id_packet_number,
            sizeof(path_id_packet_number));
   } else {