Intent to unship: getPropertyCSSValue-related interfaces Rect, RGBColor, CSSValue, CSSPrimitiveValue and CSSValueList

[Emilio Cobos Álvarez]

Related to the unshipping of getPropertyCSSValue, in bug 1459871 I
intend to unexpose the following related interfaces from the platform:

Rect, RGBColor, CSSValue, CSSPrimitiveValue, CSSValueList

Instances of these objects could only be constructed via
getPropertyCSSValue, so they're not useful in any way anymore.

Regarding the possibility of people relying on its mere presence to do
some sort of browser detection, I'd say it's quite unlikely, given we're
in this weird situation where WebKit implements and exposes them but
Blink and Edge do not.

In any case, the native code backing the interfaces will remain in tree
for a bit more, so we could always roll back if it happens to break
something.

Let me know if there's any concern with proceeding.

-- Emilio