IDL: Support using nullable on any method return type

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 71336908ed341d74bd1d874fd74ebef9069038cb..194780f409f9e4de4c88e33aadb39c801d4432e6 100644
--- a/Source/bindings/templates/methods.cpp
+++ b/Source/bindings/templates/methods.cpp
@@ -252,8 +252,7 @@ if (!{{method.cpp_value}})
     return;
 {% elif method.is_constructor %}
 {{method.cpp_type}} impl = {{cpp_value}};
-{% elif method.is_call_with_script_state or method.is_raises_exception %}
-{# FIXME: consider always using a local variable #}
+{% elif method.is_use_result_local and not method.union_arguments %}

[haraken, 2014/07/10 14:43:57]

Can we fix the code generator so that we always use the local variable (probably
before this CL)? It's a bit shame that we have to introduce use_result_local.

[Jens Widell, 2014/07/10 14:56:44]

I had that for a while. The generated code ends up uglier, but mostly that's of
no significance of course.

I think the worry is really only whether introducing an unnecessary local of
type RefPtr<> will cost in performance due to increased ref-count churn.

[haraken, 2014/07/10 15:11:44]

I think we should investigate. Since you're taking off to a vacation, I'm fine
with landing this CL first, but we definitely need to clean up the local
variable issue in the future.

[Jens Widell, 2014/07/10 15:28:53]

Very quick investigation (I just hacked my use_local_result() to always return
True):

gen/blink/bindings/modules/v8/V8EntrySync.cpp:242:57: error: conversion from
'WebCore::EntrySync*' to 'WTF::RawPtr<WebCore::DirectoryEntrySync>' is ambiguous

That's not exactly a reason not to do it, but it means it might not be 100 %
straight-forward.

 {{method.cpp_type}} result = {{cpp_value}};
 {% endif %}
 {# Post-call #}
@@ -269,7 +268,14 @@ if (exceptionState.hadException()) {
 {%- elif method.union_arguments %}
 {{union_type_method_call_and_set_return_value(method)}}
 {%- elif v8_set_return_value %}
+{% if method.is_nullable %}
+if (result.isNull())
+    v8SetReturnValueNull(info);
+else
+    {{v8_set_return_value}};
+{% else %}
 {{v8_set_return_value}};
+{% endif %}
 {%- endif %}{# None for void #}
 {# Post-set #}
 {% if interface_name == 'EventTarget' and method.name in ('addEventListener',