JS::HandleValue is basically a wrapper around a JS::Value that is safe
for exact stack rooting and moving GC.
I've tried to keep this changeset rather small and isolated and
therefore create additional JS::Rooted<T> values at some places where
the function should eventually directly take a JS::Handle<T>.
The functions "CallFunction" and "CallFunctionVoid" put their arguments
inside a JS::AutoValueVector because this will be passed directly to
"CallFunction_" with ESR31.
Refs #2462
Refs #2415
This was SVN commit r15517.