SafeDomTreeProcessor = function() {
  /** @protected @const {!Document} */
  this.inertDocument_ = createInertDocument();
}
Constructs a {@link SafeDomTreeProcessor} object that safely parses an input
string into a DOM tree using an inert document, and creates a new tree based
on the original tree, optionally transforming it in the process. The
transformation is not specified in this abstract class; subclasses are
supposed to override its protected methods to define a transformation that
allows tags and attributes, drops entire subtrees, modifies tag names or
attributes, etc.
@constructor @struct @abstract