Read WETRecord from file. Records end with 2 blank lines.

def read(cls, f):
    """Read WETRecord from file. Records end with 2 blank lines."""
    header = WETHeader.read(f)
    if header is None:
      # EOF
      return None
    content = f.read(header.length)

    # Consume empty separators
    f.readline()
    f.readline()

    return cls(header.url, content)