Combine InstanceField and StaticField and Field.

This gives us full consistency with the java.lang.reflect.Field type.

Change-Id: Ib17c7fca4fe2c912fa9f5e37da2be0aebb8def9c
diff --git a/src/class_linker.h b/src/class_linker.h
index 4720f46..edd48eb 100644
--- a/src/class_linker.h
+++ b/src/class_linker.h
@@ -56,8 +56,7 @@
   // kObjectArrayClass and kJavaLangString etc.
   Class* AllocClass();
   DexCache* AllocDexCache();
-  StaticField* AllocStaticField();
-  InstanceField* AllocInstanceField();
+  Field* AllocField();
   Method* AllocMethod();
   template <class T>
   ObjectArray<T>* AllocObjectArray(size_t length) {