[V8] Clean up CodeGeneratorV8.pm by introducing HasCustom{Getter,Setter,Method}
https://bugs.webkit.org/show_bug.cgi?id=108896

Reviewed by Adam Barth.

No tests. No change in generated code.

* bindings/scripts/CodeGeneratorV8.pm:
(GenerateHeader):
(HasCustomGetter):
(HasCustomSetter):
(HasCustomMethod):
(GetFunctionTemplateCallbackName):
(GenerateSingleBatchedAttribute):
(GenerateImplementation):
(RequiresCustomSignature):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141860 268f45cc-cd09-0410-ab3c-d52691b4dbfc