Reland Enable socketservers unittests on windows.

Original CL: https://codereview.webrtc.org/2199193004/
Revert CL: https://codereview.webrtc.org/2209333002/

NOTRY=true
BUG=webrtc:6167,webrtc:6178

Review-Url: https://codereview.webrtc.org/2218853002
Cr-Commit-Position: refs/heads/master@{#13654}

webrtc/base/socket_unittest.h

@@ -65,7 +65,7 @@ class SocketTest : public testing::Test {
 
  protected:
   void TcpInternal(const IPAddress& loopback, size_t data_size,
-      ssize_t max_send_size);
+      ptrdiff_t max_send_size);
 
  private:
   void ConnectInternal(const IPAddress& loopback);