IDL: Support optional argument default value syntax

Source/bindings/templates/methods.cpp

 {##############################################################################}
 {% macro generate_method(method, world_suffix) %}
 {% filter conditional(method.conditional_string) %}
 static void {{method.name}}{{method.overload_index}}Method{{world_suffix}}(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
     {# Local variables #}
     {% if method.has_exception_state %}
     ExceptionState exceptionState(ExceptionState::ExecutionContext, "{{method.name}}", "{{interface_name}}", info.Holder(), info.GetIsolate());
     {% endif %}
     {# Overloaded methods have length checked during overload resolution #}
@@ skipping 66 matching lines @@
 {% macro generate_arguments(method, world_suffix) %}
 {% for argument in method.arguments %}
 {{generate_argument_var_declaration(argument)}};
 {% endfor %}
 {
     {% if method.arguments_need_try_catch %}
     v8::TryCatch block;
     V8RethrowTryCatchScope rethrow(block);
     {% endif %}
     {% for argument in method.arguments %}
+    {% if argument.default_value %}

[Nils Barth (inactive), 2014/06/04 05:22:24]

This is a behavior change, right?

Could you put this in a followup, having this CL just change the syntax and
parsing?

[Jens Widell, 2014/06/04 06:12:18]

Along with the IDL file changes, yes. Although I believe I changed the IDL files
such that the end result is the same.
 
That would leave the idl_definitions.cpp change, and possibly some of the
changes in v8_methods.py and some of the additions in TestObject.idl, yes?

[Nils Barth (inactive), 2014/06/04 06:33:36]

The generated code is different (the object code may be the same);
compare old:
voidMethodDefaultNullStringStringArg
with new:
voidMethodDefaultNullStringArg
in:
https://codereview.chromium.org/312683005/diff/1/Source/bindings/tests/result...

Notably you replaced argumentOrNull with this if clause.

So I'd prefer if in the generated code weren't different in this CL:
you could put argumentOrNull in the template here, and a FIXME
to replace it with an if clause.
...and also ok to change "[Default=NullString] ..." to "... = null" in the real
IDL files, if there is no change in output.

+    if (info.Length() > {{argument.index}}) {
+        {{generate_argument(method, argument, world_suffix) | indent(8)}}
+    } else {
+        {{argument.name}} = {{argument.default_value}};
+    }
+    {% else %}
     {{generate_argument(method, argument, world_suffix) | indent}}
+    {% endif %}
     {% endfor %}
 }
 {% endmacro %}
 
 
 {######################################}
 {% macro generate_argument_var_declaration(argument) %}
 {% if argument.is_callback_interface %}
 {# FIXME: remove EventListener special case #}
 {% if argument.idl_type == 'EventListener' %}
 RefPtr<{{argument.idl_type}}> {{argument.name}}
 {%- else %}
 OwnPtr<{{argument.idl_type}}> {{argument.name}}
 {%- endif %}{# argument.idl_type == 'EventListener' #}
 {%- elif argument.is_clamp %}{# argument.is_callback_interface #}
 {# NaN is treated as 0: http://www.w3.org/TR/WebIDL/#es-type-mapping #}
 {{argument.cpp_type}} {{argument.name}} = 0
 {%- else %}
 {{argument.cpp_type}} {{argument.name}}
 {%- endif %}
 {% endmacro %}
 
 
 {######################################}
 {% macro generate_argument(method, argument, world_suffix) %}
-{% if argument.is_optional and not argument.has_default and
+{% if argument.is_optional and
+      not (argument.has_default or argument.default_value) and

[Nils Barth (inactive), 2014/06/04 05:22:24]

Can you avoid this by changing the definition in v8_methods:

from:
'has_default': 'Default' in extended_attributes,

to:
# FIXME: remove once [Default] removed and just use argument.default_value
'has_default': 'Default' in extended_attributes or argument.default_value,

[Jens Widell, 2014/06/04 06:12:18]

Will do. In general, 'has_default' and its meaning is a bit strange after this
patch; follow-ups will clean it up further, I'd expect.

[Nils Barth (inactive), 2014/06/04 06:33:36]

Yeah, at this point it becomes vestigial, and "going away soon".

       argument.idl_type != 'Dictionary' and
       not argument.is_callback_interface %}
 {# Optional arguments without a default value generate an early call with
    fewer arguments if they are omitted.
    Optional Dictionary arguments default to empty dictionary. #}
 if (UNLIKELY(info.Length() <= {{argument.index}})) {
     {% if world_suffix %}
     {{cpp_method_call(method, argument.v8_set_return_value_for_main_world, argument.cpp_value) | indent}}
     {% else %}
     {{cpp_method_call(method, argument.v8_set_return_value, argument.cpp_value) | indent}}
@@ skipping 429 matching lines @@
     {% endif %}
     {{constructor.cpp_type}} impl = {{cpp_class}}::createForJSConstructor({{constructor.argument_list | join(', ')}});
     {% if is_constructor_raises_exception %}
     if (exceptionState.throwIfNeeded())
         return;
     {% endif %}
 
     {{generate_constructor_wrapper(constructor) | indent}}
 }
 {% endmacro %}