Add missing references.

This mainly avoids implicit copies when extracting an object from a container.

Change-Id: If4d0e6153e8c2b48345fde5bb546b4c65649fcf3
diff --git a/runtime/stack.h b/runtime/stack.h
index 700b7f1..a4b93bc 100644
--- a/runtime/stack.h
+++ b/runtime/stack.h
@@ -567,7 +567,7 @@
   static void DescribeStack(Thread* thread) SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
  private:
-  instrumentation::InstrumentationStackFrame GetInstrumentationStackFrame(uint32_t depth) const;
+  instrumentation::InstrumentationStackFrame& GetInstrumentationStackFrame(uint32_t depth) const;
 
   void SanityCheckFrame() const SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);