bind-gen: Fix [Unforgeable] and promise-returning functions

Bug: 839389
Change-Id: I95db56fa113354486742149024cd34173364eb46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142074Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758435}

third_party/blink/renderer/bindings/scripts/bind_gen/codegen_context.py

@@ -244,6 +244,14 @@ class CodeGenContext(object):
             return False
         return self.return_type.unwrap().is_union
 
+    @property
+    def is_return_type_promise_type(self):
+        if self.attribute:
+            return self.attribute.idl_type.unwrap().is_promise
+        if self.operation_group:
+            return self.operation_group[0].return_type.unwrap().is_promise
+        return False
+
     @property
     def may_throw_exception(self):
         if not self.member_like:

third_party/blink/renderer/bindings/scripts/bind_gen/interface.py

@@ -335,7 +335,7 @@ def bind_callback_local_vars(code_node, cg_context):
     if cg_context.property_ and cg_context.property_.identifier:
         _1.append("${property_name}")
     _2 = ""
-    if cg_context.return_type and cg_context.return_type.unwrap().is_promise:
+    if cg_context.is_return_type_promise_type:
         _2 = ("\n"
               "ExceptionToRejectPromiseScope reject_promise_scope"
               "(${info}, ${exception_state});")
@@ -789,7 +789,7 @@ def make_check_receiver(cg_context):
                 body=T("return;")),
         ])
 
-    if cg_context.return_type and cg_context.return_type.unwrap().is_promise:
+    if cg_context.is_return_type_promise_type:
         return SequenceNode([
             T("// Promise returning function: "
               "Convert a TypeError to a reject promise."),
@@ -3679,10 +3679,16 @@ def _make_property_entry_v8_property_attribute(property_):
     if "NotEnumerable" in property_.extended_attributes:
         values.append("v8::DontEnum")
     if "Unforgeable" in property_.extended_attributes:
+        if not isinstance(property_, web_idl.Attribute):
+            values.append("v8::ReadOnly")
         values.append("v8::DontDelete")
     if not values:
         values.append("v8::None")
-    return "static_cast<v8::PropertyAttribute>({})".format(" | ".join(values))
+    if len(values) == 1:
+        return values[0]
+    else:
+        return "static_cast<v8::PropertyAttribute>({})".format(
+            " | ".join(values))
 
 
 def _make_property_entry_on_which_object(property_):