Auto merge of #14464 - canaltinova:cssom-test, r=Manishearth

Write tests for CSSOM Interfaces

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ToCss implementation was wrong about Keyframe percentage values, it was writing values between 0-1. I had to fix it. Wrote some tests about CSSKeyframesRule, CSSNamespaceRule, CSSRuleList, CSSStyleSheet, StyleSheetList interfaces.
CSSFontFaceRule and CSSViewportRule looks like not implemented yet. I didn't write one for them.
Also name attribute in CSSKeyframesRule isn't implemented yet. Is there any complication about it? If not, I can implement it.

r? @Manishearth

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- [X] There are tests for these changes OR

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