IDL: Support optional argument default value syntax

Source/bindings/templates/methods.cpp

Index: Source/bindings/templates/methods.cpp
diff --git a/Source/bindings/templates/methods.cpp b/Source/bindings/templates/methods.cpp
index f1f0fdb5593b5155d4e13acf141d648f200f3ca3..113fdd78f0c66158090606c287829ddc876822fa 100644
--- a/Source/bindings/templates/methods.cpp
+++ b/Source/bindings/templates/methods.cpp
@@ -84,7 +84,15 @@ if (listener && !impl->toNode())
     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 %}
@@ -110,7 +118,8 @@ OwnPtr<{{argument.idl_type}}> {{argument.name}}
 
 {######################################}
 {% 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