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/mirror/art_method.cc b/runtime/mirror/art_method.cc
index fe27992..6b897cb 100644
--- a/runtime/mirror/art_method.cc
+++ b/runtime/mirror/art_method.cc
@@ -320,6 +320,15 @@
   self->PopManagedStackFragment(fragment);
 }
 
+#ifndef NDEBUG
+size_t ArtMethod::GetSirtOffsetInBytes() {
+  CHECK(IsNative());
+  // TODO: support Sirt access from generic JNI trampoline.
+  CHECK_NE(GetEntryPointFromQuickCompiledCode(), GetQuickGenericJniTrampoline());
+  return kPointerSize;
+}
+#endif
+
 bool ArtMethod::IsRegistered() {
   void* native_method =
       GetFieldPtr<void*>(OFFSET_OF_OBJECT_MEMBER(ArtMethod, entry_point_from_jni_), false);