Remove static GcRoot<>s from Class and ClassExt.

And clean up gc_root includes in runtime/mirror/.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: Ib5c42a3a892ced4440720350a63a94bcf3a1ca75
diff --git a/runtime/proxy_test.h b/runtime/proxy_test.h
index 860d96c..411dc7a 100644
--- a/runtime/proxy_test.h
+++ b/runtime/proxy_test.h
@@ -40,11 +40,10 @@
                                          const std::vector<Handle<mirror::Class>>& interfaces)
     REQUIRES_SHARED(Locks::mutator_lock_) {
   StackHandleScope<1> hs(soa.Self());
-  Handle<mirror::Class> javaLangObject = hs.NewHandle(
-      class_linker->FindSystemClass(soa.Self(), "Ljava/lang/Object;"));
+  Handle<mirror::Class> javaLangObject = hs.NewHandle(GetClassRoot<mirror::Object>());
   CHECK(javaLangObject != nullptr);
 
-  jclass javaLangClass = soa.AddLocalReference<jclass>(mirror::Class::GetJavaLangClass());
+  jclass javaLangClass = soa.AddLocalReference<jclass>(GetClassRoot<mirror::Class>());
 
   // Builds the interfaces array.
   jobjectArray proxyClassInterfaces =