Bug 1100580 part 1.   Drop support for having an off-thread compilation token and wanting a script rval from nsJSUtils::EvaluateString, since we never need that in practice.  r=bholley
Bug 1100580 part 2.  Get rid of EvaluateOptions::needResult, since we can use JS::CompileOptions::noScriptRval (with the opposite meaning, but same default behavior) for this purpose.  r=bholley