ART: Add GetLoadedClasses support

Add support to retrieve all loaded classes. Use the standard
ClassLoader callback infrastructure.

Bug: 31684578
Test: m test-art-host-run-test-907-get-loaded-classes
Change-Id: Ifb33cae27dc32602bcd8fd930d3c7eda02598c26
diff --git a/runtime/openjdkjvmti/OpenjdkJvmTi.cc b/runtime/openjdkjvmti/OpenjdkJvmTi.cc
index 05da585..ac8d5e1 100644
--- a/runtime/openjdkjvmti/OpenjdkJvmTi.cc
+++ b/runtime/openjdkjvmti/OpenjdkJvmTi.cc
@@ -476,7 +476,8 @@
   }
 
   static jvmtiError GetLoadedClasses(jvmtiEnv* env, jint* class_count_ptr, jclass** classes_ptr) {
-    return ERR(NOT_IMPLEMENTED);
+    HeapUtil heap_util(&gObjectTagTable);
+    return heap_util.GetLoadedClasses(env, class_count_ptr, classes_ptr);
   }
 
   static jvmtiError GetClassLoaderClasses(jvmtiEnv* env,