Adds a function for encoding a Merkle tree leaf in TLS wire format.

net/cert/merkle_tree_leaf.h

Index: net/cert/merkle_tree_leaf.h
diff --git a/net/cert/merkle_tree_leaf.h b/net/cert/merkle_tree_leaf.h
index efbe185713c6879afbe89c79ebfeb377e48f70fc..2a69bc4a3a77abac4121a6f0ff39056d09ff6825 100644
--- a/net/cert/merkle_tree_leaf.h
+++ b/net/cert/merkle_tree_leaf.h
@@ -22,9 +22,17 @@ namespace ct {
 // Has all the data as the MerkleTreeLeaf defined in the RFC, arranged
 // slightly differently.
 struct NET_EXPORT MerkleTreeLeaf {
+  // Version enum in RFC 6962, Section 3.2.

[Eran Messeri, 2016/05/04 10:29:23]

Nit: I'd inline those in ct_serialization.cc and avoid changing
merkle_tree_leaf.h entirely.
You'd point out that we did this differently for SignedCertificateTimestamp -
and you're right - but now I'm fairly confident there won't be another version /
type of MerkleTreeLeaf for RFC6962.
Since we only care about these values during serialization, then it's simpler to
have them in the serialization code.

[Rob Percival, 2016/05/05 15:45:31]

Done.

+  enum Version { VERSION_1 = 0, };
+  // MerkleLeafType enum in RFC 6962, Section 3.4.
+  enum Type { TYPE_TIMESTAMPED_ENTRY = 0, };
+
   MerkleTreeLeaf();
   ~MerkleTreeLeaf();
 
+  // Version and Merkle leaf type fields are omitted, as there is only one
+  // possible value for each of them in CTv1.
+
   // The log id this leaf belongs to.
   std::string log_id;