instantiateJSToNativeConversion should work directly with JSToNativeConversion class

[jdm]

Instead of lots of code taking the result of getJSToNativeConversion and extracting the members of the JSToNativeConversion class to pass as arguments to instantiateJSToNativeConversion, it will be easier to modify how those methods work if instantiateJSToNativeConversion takes the class directly as an argument. This should allow cleaning up the callers that pass the isAutoRooted argument, since that can be computed by getJSToNativeConversion and stored in the class. This should allow more consistent handling of types that should be stored in a CustomAutoRoot value.

[highfive]

cc @wafflespeanut