Add client certificates to HttpsServer and HttpClient.

runtime/bin/secure_socket.h

Index: runtime/bin/secure_socket.h
diff --git a/runtime/bin/secure_socket.h b/runtime/bin/secure_socket.h
index f20ca7d65a75930d60dda594442bfcc6a3b7b919..84bbbb5fc7b524ab79800399b14e58a54d96455f 100644
--- a/runtime/bin/secure_socket.h
+++ b/runtime/bin/secure_socket.h
@@ -68,6 +68,7 @@ class SSLFilter {
         handshake_complete_(NULL),
         bad_certificate_callback_(NULL),
         in_handshake_(false),
+        client_certificate_name_(NULL),
         filter_(NULL) { }
 
   void Init(Dart_Handle dart_this);
@@ -104,6 +105,7 @@ class SSLFilter {
   Dart_Handle bad_certificate_callback_;
   bool in_handshake_;
   bool is_server_;
+  char* client_certificate_name_;
   PRFileDesc* filter_;
 
   void InitializeBuffers(Dart_Handle dart_this);