JNI down call fixes.

Ensure SIRT isn't accessed via quick callee save frame.
Some tidying of code.

Change-Id: I8fec3e89aa6d2e86789c60a07550db2e92478ca7
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 6d8ede5..680d269 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -1829,11 +1829,11 @@
   return result;
 }
 
-struct CurrentMethodVisitor : public StackVisitor {
+struct CurrentMethodVisitor FINAL : public StackVisitor {
   CurrentMethodVisitor(Thread* thread, Context* context)
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_)
       : StackVisitor(thread, context), this_object_(nullptr), method_(nullptr), dex_pc_(0) {}
-  virtual bool VisitFrame() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+  bool VisitFrame() OVERRIDE SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
     mirror::ArtMethod* m = GetMethod();
     if (m->IsRuntimeMethod()) {
       // Continue if this is a runtime method.