Implement XHR progress event changes
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This PR implements change in the specs in XHR.
Unfortunatelly, @jdm the test are not passing. It looks like the fetch module send more bytes that we should according the ```Content-Length```. This [lines](https://github.com/servo/servo/blob/master/tests/wpt/web-platform-tests/XMLHttpRequest/progress-events-response-data-gzip.htm#L70) make the test fail.
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Trigger reflow on document.elementsFromPoint
As [suggested](https://github.com/servo/servo/issues/15592#issuecomment-280379805) by @jdm `Document::nodes_from_point` now triggers a reflow.
I added a new reftest that panics with `ERROR:servo: Tried to hit test without a DisplayList` if this patch is not applied.
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- [X] These changes fix#15592.
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Shorthand cannot represent CSS-wide keyword in individual longhands, and
thus we cannot serialize longhand with CSS-wide keyword as part of a
shorthand.
fix {transform,perspective}-origin accepts (and ignores) anything for…
… their second part of value
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Background serialization of multiple values.
Similar to animation, the serialization of multiple backgrounds should
return the longhands serialization if there are an unequal number of
values for the required properties. This is part of #15398
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