Indexed getter for readonly CSSStyleDeclaration is broken

[jdm]

window.getComputedStyle(document.documentElement)[0] returns align-content in Firefox, but undefined in Servo. get_property_value correctly has a different codepath for readonly (ie. computed) style declarations, but IndexedGetter (and Length) do not. It shouldn't be too hard to reuse the get_computed_style code path for the indexed getter as long as we can figure out how to map the array indices to property names, but it's not totally clear how to get the length of the computed styles.

This also has the effect that Object.getOwnPropertyNames(window.getComputedStyle(document.documentElement)) returns an empty object, and this ends up breaking the MotionMark benchmark's browser detection.

[jdm]

This should be less work than I initially thought, since c10e839 made it easier to access style data for a node. We'll need the following:

• an equivalent for https://searchfox.org/mozilla-central/source/layout/style/GenerateComputedDOMStyleGenerated.py that generates a map of indexes to CSS properties
• use that generated map for the IndexedGetter and Length methods in CSSStyleDeclaration for readonly declarations

It probably makes sense to generate this map in the same way that we generate CSSStyleDeclaration.webidl, based on the output of the style crate's CSS property generation.