Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/stack.cc b/runtime/stack.cc
index 0cdc984..0adf031 100644
--- a/runtime/stack.cc
+++ b/runtime/stack.cc
@@ -127,7 +127,8 @@
     }
   } else if (m->IsOptimized()) {
     // TODO: Implement, currently only used for exceptions when jdwp is enabled.
-    LOG(WARNING) << "StackVisitor::GetThisObject is unimplemented with the optimizing compiler";
+    UNIMPLEMENTED(WARNING)
+        << "StackVisitor::GetThisObject is unimplemented with the optimizing compiler";
     return nullptr;
   } else {
     const DexFile::CodeItem* code_item = m->GetCodeItem();