[SysApps] raw_socket: Initialize |write_buffer_size_|.

The member variable was not being initialized in the constructor, so in
the worst case UDPSocketObject::OnSend() could be called with a bogus
value there.

CID=194757
BUG=XWALK-2928
Merge pull request #2613 from rakuco/CID-194757

[SysApps] raw_socket: Initialize |write_buffer_size_|.