Bug 997285 part 1. Add JS_GetErrorPrototype to JSAPI. r=jorendorff

js/src/jsapi.cpp

@@ -1384,6 +1384,14 @@ JS_GetArrayPrototype(JSContext *cx, HandleObject forObj)
     return GlobalObject::getOrCreateArrayPrototype(cx, global);
 }
 
+JS_PUBLIC_API(JSObject *)
+JS_GetErrorPrototype(JSContext *cx)
+{
+    CHECK_REQUEST(cx);
+    Rooted<GlobalObject*> global(cx, cx->global());
+    return GlobalObject::getOrCreateCustomErrorPrototype(cx, global, JSEXN_ERR);
+}
+
 JS_PUBLIC_API(JSObject *)
 JS_GetGlobalForObject(JSContext *cx, JSObject *obj)
 {

js/src/jsapi.h

@@ -1826,6 +1826,13 @@ JS_GetObjectPrototype(JSContext *cx, JS::HandleObject forObj);
 extern JS_PUBLIC_API(JSObject *)
 JS_GetArrayPrototype(JSContext *cx, JS::HandleObject forObj);
 
+/*
+ * Returns the original value of |Error.prototype| from the global
+ * object of the current compartment of cx.
+ */
+extern JS_PUBLIC_API(JSObject *)
+JS_GetErrorPrototype(JSContext *cx);
+
 extern JS_PUBLIC_API(JSObject *)
 JS_GetGlobalForObject(JSContext *cx, JSObject *obj);