CodegenRust fixes for NamedGetter
Added Element::set_custom_attribute
Implement HTMLElement.dataset

Make DOMStringMap use related Element's custom attributes values.
Avoid duplicated code
Update web-platform-tests results
Use the try macro in unwrap_jsmanaged.
Remove the proto_{id,depth} arguments from unwrap_jsmanaged.

Instead, we infer them from the type we're unwrapping into. This will prevent
any mismatches between the type we return and the type we check for.
Use unwrap() when finalizing.

This allows us to duplicate a little less code.
Move unwrap_jsmanaged and related machinery to conversions.rs.
Remove the unused DOMRefCell::unwrap method.
Document DOMRefCell.

Based on the RefCell documentation.
auto merge of #4449 : Ms2ger/servo/unwrap_jsmanaged, r=jdm
Update HTML{Div,Span,}Element stack size
auto merge of #4038 : brunoabinader/servo/dataset, r=Ms2ger

Spec: https://html.spec.whatwg.org/multipage/dom.html#dom-dataset

Closes #2974.
auto merge of #4450 : Ms2ger/servo/domrefcell-doc, r=jdm
Merge branch 'master' into dispatch-mousemove