Revert "Revert "Add allocator field to ClassLoader.""

Bug: 22720414

This reverts commit 06419efad783a139bd1d9e28cd12b2fa691d3e93.
diff --git a/libart/src/main/java/java/lang/ClassLoader.java b/libart/src/main/java/java/lang/ClassLoader.java
index 860e4a1..4de34cd 100644
--- a/libart/src/main/java/java/lang/ClassLoader.java
+++ b/libart/src/main/java/java/lang/ClassLoader.java
@@ -92,6 +92,12 @@
     private Map<String, Package> packages = new HashMap<String, Package>();
 
     /**
+     * Pointer to the allocator used by the runtime to allocate metadata such
+     * as ArtFields and ArtMethods.
+     */
+    private transient long allocator;
+
+    /**
      * Pointer to the class table, only used from within the runtime.
      */
     private transient long classTable;