registerProperty/unregisterProperty should be clearer about escaping of property name

[dbaron]

The definition of registerProperty() and unregisterProperty() should be clearer about CSS escaping of the property name. It says that the name given has to match a CSS production that does include escaping, but it doesn't actually say that that CSS escaping is processed in the production of the property name that is actually registered, or that registering and unregistering by different strings that are equivalent once escaping is processed properly unregisters, or similar for registering twice throwing an exception.

[tabatkins]

It looks like we don't currently CSS-parse it, and instead just take it literally.

If you run:

CSS.registerProperty({
   name:"--f\\30o", 
   syntax:"<color>", 
   initialValue:"blue", 
   inherits:false});

then getComputedStyle(document.body).getPropertyValue("--f\\30o") returns blue, while getComputedStyle(document.body).getPropertyValue("--f0o") returns the empty string.

So yeah, I'm fine with specifying, like in other parts of the platform and TypedOM, that the name is taken literally.

[tabatkins]

(Done in ec02741)

[tabatkins]

(and also in 17d9a30 just now for additional cleanup)