layout: Make `box-sizing` only affect explicitly-specified or percentage
sizes, not automatically computed ones.

Improves GitHub significantly.
auto merge of #3615 : pcwalton/servo/box-sizing-fix, r=glennw

Improves GitHub significantly.

r? @glennw
Add #[privatize] to allow enforcing private fields for struct definitions
Simple privatizations
Privatize InheritTypes
Privatize Attr
Privatize Comment
Privatize DocumentType
Privatize EventTarget and EventListenerEntry
Privatize File
Privatize ProcessingInstruction
Privatize Document
Privatize Window
Privatize Node
Privatize Event
Privatize Element