From https://dom.spec.whatwg.org/#concept-getelementsbytagname:
2. Otherwise, if root’s node document is an HTML document, return a
HTMLCollection rooted at root, whose filter matches the following
descendant elements:
* Whose namespace is the HTML namespace and whose qualified name
is qualifiedName, in ASCII lowercase.
* Whose namespace is not the HTML namespace and whose qualified
name is qualifiedName.
This patch adds an IPC call for debugging requests. It's stringly typed
and very simple, and allows us to easily implement all the features in
the Browser's Debug menu.
