Auto merge of #365 - cbrewster:callargs_new_target, r=jdm
Auto merge of #365 - cbrewster:callargs_new_target, r=jdm

Add new_target method to CallArgs

Needed to retrieve newTarget during construction for HTML Constructors:

https://html.spec.whatwg.org/multipage/dom.html#htmlconstructor

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servo/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs,nnethercote/rust-mozjs,nnethercote/rust-mozjs,nnethercote/rust-mozjs,servo/rust-mozjs,nnethercote/rust-mozjs,nnethercote/rust-mozjs