Remove the help text for render backend option. follow up #3711.
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.
Reenable visibilty ref test. Fixes #1576.
Convert internal font code to store pt size as Au.
auto merge of #3742 : saneyuki/servo/opt, r=jdm

This follows up #3711.
auto merge of #3768 : glennw/servo/fix-1576, r=jdm
auto merge of #3769 : glennw/servo/font-au, r=pcwalton