Bug 959012 - When resolving iterator classes, use JSProto_Iterator instead of JSProto_GeneratorFunction. r=luke

The former triggers all of the appropriate initialization, whereas the latter
is 'imaginary' in jsprototypes.h, which means that the is no initialization
function associated with it.
Bug 959012 - Remove unused cx parameter from JS_IdentifyClassPrototype. r=jorendorff
Bug 959012 - Move js_GetClassObject creation logic into GlobalObject. r=jorendorff
Bug 959012 - Create separate js_GetClassPrototype and js_FindClassPrototype to mimic js_{Get,Find}ClassObject. r=jorendorff

The current behavior is crazy, and most definitely not Xray-safe.
Bug 959012 - Clean up and separate the semantics of js_{Get,Find}Class{Prototype,Object}. r=jorendorff