Bug 860494 - Make resolveNativeProperty a virtual instance method in XrayTraits like resolveOwnProperty. r=bz

The current setup is just an artifact of how it used to be before I refactored
Xrays. Have it as a virtual trap is more flexible since it allows us to invoke
the right trap by just calling GetXrayTraits(wrapper) from non-templatized code.
Bug 860494 - Clarify the semantics of XrayTraits::resolveOwnProperty. r=bz

Right now, it sometimes fills out |desc|, and sometimes just defines the property
on the holder. This can get confusing, so let's refine the semantics here and
describe them in a big comment.
Bug 860494 - Check for native properties before checking named children on XOWs. r=bz
Bug 860494 - Move the named property check further down in XrayWrapper. r=bz
Bug 863766 - crash when destroying a JSContext with outstanding requests. r=luke