API functions returning Promises should not throw exceptions.

Source/bindings/scripts/v8_types.py

Index: Source/bindings/scripts/v8_types.py
diff --git a/Source/bindings/scripts/v8_types.py b/Source/bindings/scripts/v8_types.py
index e5d9a09e402a80733d8bd6e2c522caad4a8ec9ba..cbd964ce593135c0c1bfce32e4475c3909909d0f 100644
--- a/Source/bindings/scripts/v8_types.py
+++ b/Source/bindings/scripts/v8_types.py
@@ -414,7 +414,28 @@ def v8_value_to_local_cpp_value(idl_type, extended_attributes, v8_value, variabl
     cpp_value = v8_value_to_cpp_value(idl_type, extended_attributes, v8_value, index)
     return format_string.format(cpp_type=this_cpp_type, cpp_value=cpp_value, variable_name=variable_name)
 
+
+def v8_value_to_local_cpp_value_async(idl_type, extended_attributes, v8_value, variable_name, index=None):
+    """Returns an expression that converts a V8 value to a C++ value and stores it as a local value.
+    The generated code returns a rejected Promise when it sees an error rather than throwing an exception.
+    """
+    this_cpp_type = idl_type.cpp_type_args(extended_attributes=extended_attributes, used_as_argument=True)
+
+    idl_type = idl_type.preprocessed_type
+
+    if idl_type.base_type == 'DOMString' and not idl_type.array_or_sequence_type:
+        format_string = 'V8TRYCATCH_FOR_V8STRINGRESOURCE_VOID_PROMISE({cpp_type}, {variable_name}, {cpp_value}, info)'
+    elif idl_type.is_integer_type:
+        format_string = 'V8TRYCATCH_EXCEPTION_VOID_PROMISE({cpp_type}, {variable_name}, {cpp_value}, info, exceptionState)'
+    else:
+        format_string = 'V8TRYCATCH_VOID_PROMISE({cpp_type}, {variable_name}, {cpp_value}, info)'
+
+    cpp_value = v8_value_to_cpp_value(idl_type, extended_attributes, v8_value, index)
+    return format_string.format(cpp_type=this_cpp_type, cpp_value=cpp_value, variable_name=variable_name)
+
+
 IdlType.v8_value_to_local_cpp_value = v8_value_to_local_cpp_value
+IdlType.v8_value_to_local_cpp_value_async = v8_value_to_local_cpp_value_async
 IdlUnionType.v8_value_to_local_cpp_value = v8_value_to_local_cpp_value