Refactor ClassRoot/GetClassRoot().

Move it outside the ClassLinker, into its own header file,
and add retrieval based on a mirror class template argument.
Keep the SetClassRoot() as a private member of ClassLinker.

Make the new GetClassRoot()s return ObjPtr<>.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: Icbc6b62b41f6ffd65b437297a21eadbb0454e2b7
diff --git a/runtime/class_root.cc b/runtime/class_root.cc
new file mode 100644
index 0000000..08820b0
--- /dev/null
+++ b/runtime/class_root.cc
@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "class_root.h"
+
+namespace art {
+
+const char* GetClassRootDescriptor(ClassRoot class_root) {
+  static const char* class_roots_descriptors[] = {
+#define CLASS_ROOT_DESCRIPTOR(name, descriptor, mirror_type) descriptor,
+      CLASS_ROOT_LIST(CLASS_ROOT_DESCRIPTOR)
+#undef CLASS_ROOT_DESCRIPTOR
+  };
+  static_assert(arraysize(class_roots_descriptors) == static_cast<size_t>(ClassRoot::kMax),
+                "Mismatch between class descriptors and class-root enum");
+
+  DCHECK_LT(static_cast<uint32_t>(class_root), static_cast<uint32_t>(ClassRoot::kMax));
+  const char* descriptor = class_roots_descriptors[static_cast<size_t>(class_root)];
+  CHECK(descriptor != nullptr);
+  return descriptor;
+}
+
+}  // namespace art