Tidy MethodProtoHelper.

Move to place of only use, class_linker.cc. Be lazy in computing the name.

Change-Id: I1438efbda58369ddd0ac36eda8a5a0a6c6fdff77
diff --git a/runtime/mirror/art_method-inl.h b/runtime/mirror/art_method-inl.h
index cb6ac4f..a0b6adf 100644
--- a/runtime/mirror/art_method-inl.h
+++ b/runtime/mirror/art_method-inl.h
@@ -436,11 +436,15 @@
   return GetInterfaceMethodIfProxy()->GetDeclaringClass()->GetDexCache();
 }
 
+inline bool ArtMethod::IsProxyMethod() {
+  return GetDeclaringClass()->IsProxyClass();
+}
+
 inline ArtMethod* ArtMethod::GetInterfaceMethodIfProxy() {
-  mirror::Class* klass = GetDeclaringClass();
-  if (LIKELY(!klass->IsProxyClass())) {
+  if (LIKELY(!IsProxyMethod())) {
     return this;
   }
+  mirror::Class* klass = GetDeclaringClass();
   mirror::ArtMethod* interface_method = GetDexCacheResolvedMethods()->Get(GetDexMethodIndex());
   DCHECK(interface_method != nullptr);
   DCHECK_EQ(interface_method,