Pepper: Add a test for PPB_TCPSocket_Private.

ppapi/cpp/private/flash_tcp_socket.cc

Index: ppapi/cpp/private/flash_tcp_socket.cc
diff --git a/ppapi/cpp/private/flash_tcp_socket.cc b/ppapi/cpp/private/flash_tcp_socket.cc
index 0dec2181d90ca947d5b080df3ecaad6a882b460f..18c865f1420dc5715bbead2ba04c3b0cad0afbc2 100644
--- a/ppapi/cpp/private/flash_tcp_socket.cc
+++ b/ppapi/cpp/private/flash_tcp_socket.cc
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// TODO(yzshen): See the comment in corresponding .h file.
-
 #include "ppapi/cpp/private/flash_tcp_socket.h"
 
 #include "ppapi/c/pp_bool.h"
@@ -32,6 +30,11 @@ TCPSocket::TCPSocket(Instance* instance) {
   }
 }
 
+// static
+bool TCPSocket::IsAvailable() {
+  return has_interface<PPB_Flash_TCPSocket>();
+}
+
 int32_t TCPSocket::Connect(const char* host,
                            uint16_t port,
                            const CompletionCallback& callback) {