API functions returning Promises should not throw exceptions.

Source/bindings/scripts/v8_methods.py

Index: Source/bindings/scripts/v8_methods.py
diff --git a/Source/bindings/scripts/v8_methods.py b/Source/bindings/scripts/v8_methods.py
index 69ba2c1239e59e01df2889ca92611b6780aa739d..3f696d7ea86aab5011f4adc28dbe1a6fe00f163e 100644
--- a/Source/bindings/scripts/v8_methods.py
+++ b/Source/bindings/scripts/v8_methods.py
@@ -174,6 +174,7 @@ def generate_argument(interface, method, argument, index):
         'v8_set_return_value_for_main_world': v8_set_return_value(interface.name, method, this_cpp_value, for_main_world=True),
         'v8_set_return_value': v8_set_return_value(interface.name, method, this_cpp_value),
         'v8_value_to_local_cpp_value': v8_value_to_local_cpp_value(argument, index),
+        'v8_value_to_local_cpp_value_async': v8_value_to_local_cpp_value_async(argument, index),
     }
 
 
@@ -255,6 +256,24 @@ def v8_value_to_local_cpp_value(argument, index):
                                                 v8_value, name, index=index)
 
 
+def v8_value_to_local_cpp_value_async(argument, index):
+    extended_attributes = argument.extended_attributes
+    idl_type = argument.idl_type
+    name = argument.name
+    if argument.is_variadic:
+        vector_type = 'WillBeHeapVector' if idl_type.is_will_be_garbage_collected else 'Vector'
+        return 'V8TRYCATCH_VOID_PROMISE({vector_type}<{cpp_type}>, {name}, toNativeArguments<{cpp_type}>(info, {index}), info)'.format(
+                cpp_type=idl_type.cpp_type, name=name, index=index, vector_type=vector_type)
+    # [Default=NullString]
+    if (argument.is_optional and idl_type.name == 'String' and
+        extended_attributes.get('Default') == 'NullString'):
+        v8_value = 'argumentOrNull(info, %s)' % index
+    else:
+        v8_value = 'info[%s]' % index
+    return idl_type.v8_value_to_local_cpp_value_async(argument.extended_attributes,
+                                                      v8_value, name, index=index)
+
+
 ################################################################################
 # Auxiliary functions
 ################################################################################