Improve code generator scripts to support converting ScriptString.

Improve code generator scripts to support converting ScriptString.
https://bugs.webkit.org/show_bug.cgi?id=38699

Reviewed by Adam Barth.

Change both JSC and V8 generators to introduce "ConvertScriptString"
attribute to allow converting from ScriptString. Also updated the
bindings test result.

These changes are necessary in order to avoid adding custom binding codes
when we add the FileReader interface.

* bindings/scripts/CodeGeneratorJS.pm:
* bindings/scripts/CodeGeneratorV8.pm:
* bindings/scripts/test/GObject/WebKitDOMTestObj.cpp:
(webkit_dom_test_obj_get_script_string_attr):
(webkit_dom_test_obj_get_property):
(webkit_dom_test_obj_class_init):
* bindings/scripts/test/GObject/WebKitDOMTestObj.h:
* bindings/scripts/test/JS/JSTestObj.cpp:
(WebCore::):
(WebCore::jsTestObjScriptStringAttr):
* bindings/scripts/test/JS/JSTestObj.h:
* bindings/scripts/test/ObjC/DOMTestObj.h:
* bindings/scripts/test/ObjC/DOMTestObj.mm:
(-[DOMTestObj scriptStringAttr]):
* bindings/scripts/test/TestObj.idl:
* bindings/scripts/test/V8/V8TestObj.cpp:
(WebCore::TestObjInternal::scriptStringAttrAttrGetter):
(WebCore::):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@58921 268f45cc-cd09-0410-ab3c-d52691b4dbfc