Allow embedder to set promise internal field count

src/builtins/builtins-promise-gen.cc

Index: src/builtins/builtins-promise-gen.cc
diff --git a/src/builtins/builtins-promise-gen.cc b/src/builtins/builtins-promise-gen.cc
index a050f18d668a4b591ef78a4766504f7cc7208ba3..cd4e381a1bbfbad2a3fa6c540bf5c13a1c9116e3 100644
--- a/src/builtins/builtins-promise-gen.cc
+++ b/src/builtins/builtins-promise-gen.cc
@@ -17,6 +17,7 @@ namespace internal {
 using compiler::Node;
 
 Node* PromiseBuiltinsAssembler::AllocateJSPromise(Node* context) {
+  // Does this need to change to account for new internal fields?
   Node* const native_context = LoadNativeContext(context);
   Node* const promise_fun =
       LoadContextElement(native_context, Context::PROMISE_FUNCTION_INDEX);
@@ -31,6 +32,10 @@ void PromiseBuiltinsAssembler::PromiseInit(Node* promise) {
                                  SmiConstant(v8::Promise::kPending));
   StoreObjectFieldNoWriteBarrier(promise, JSPromise::kFlagsOffset,
                                  SmiConstant(0));
+  for (int i = 0; i < v8::Promise::kEmbedderFieldCount; i++) {
+    int offset = JSPromise::kSize + i * kPointerSize;
+    StoreObjectFieldNoWriteBarrier(promise, offset, SmiConstant(Smi::kZero));
+  }
 }
 
 Node* PromiseBuiltinsAssembler::AllocateAndInitJSPromise(Node* context) {