Use DOMRefCell for Document.
Use DOMRefCell for Event.
Use DOMRefCell for EventTarget.
Use DOMRefCell for FormData.
Use DOMRefCell for HTMLImageElement.
Use DOMRefCell for HTMLInputElement.
Use DOMRefCell for Node.

Altough LayoutDataRef is touched, we don't use DOMRefCell in it becasuse
it's expected to manipulate in other task.
Use DOMRefCell for Window.
Use DOMRefCell for ServoHTMLParser.
Use DOMRefCell for URLSearchParams.
Use DOMRefCell for XMLHttpRequest.
Use DOMRefCell for TimerManager.
Use DOMRefCell for ScriptTask.
Use DOMRefCell for Page.
auto merge of #3737 : saneyuki/servo/cell, r=jdm

#3050

Altough LayoutDataRef is touched from layout, we don't use DOMRefCell in it becasuse
it's expected to manipulate in layout task.
Usage of JSRef<Attr> in before_remove_attr & after_set_attr

JSRef<Attr> does not require allocating a DOMString for value, which are
unused in most cases. It also provides more access to Attr data.