Add a DebuggedSourceFile proto.

def add_debugged_source_file(self, debugged_source_file):
    """Add a DebuggedSourceFile proto."""
    # TODO(cais): Should the key include a host name, for certain distributed
    #   cases?
    key = debugged_source_file.file_path
    self._source_file_host[key] = debugged_source_file.host
    self._source_file_last_modified[key] = debugged_source_file.last_modified
    self._source_file_bytes[key] = debugged_source_file.bytes
    self._source_file_content[key] = debugged_source_file.lines