Implement Promise.prototype.then and Promise.prototype.catch

Source/core/dom/Promise.idl

Index: Source/core/dom/Promise.idl
diff --git a/Source/core/dom/Promise.idl b/Source/core/dom/Promise.idl
index 893b8eefcf88606ebbdc2d59962a106e074795a6..7460d95134bd7680f75b211c07d869d1949dd9a7 100644
--- a/Source/core/dom/Promise.idl
+++ b/Source/core/dom/Promise.idl
@@ -32,4 +32,6 @@
    CustomConstructor(any init),
    EnabledAtRuntime=promise
 ] interface Promise {
+   [Custom] Promise then(optional any fulfillCallback, optional any rejectCallback);
+   [Custom] Promise catch(optional any rejectCallback);
 };