Add missing references.

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

Change-Id: If4d0e6153e8c2b48345fde5bb546b4c65649fcf3
diff --git a/runtime/stack.cc b/runtime/stack.cc
index 1715664..5d3a9a5 100644
--- a/runtime/stack.cc
+++ b/runtime/stack.cc
@@ -253,7 +253,7 @@
   return result;
 }
 
-instrumentation::InstrumentationStackFrame StackVisitor::GetInstrumentationStackFrame(uint32_t depth) const {
+instrumentation::InstrumentationStackFrame& StackVisitor::GetInstrumentationStackFrame(uint32_t depth) const {
   return thread_->GetInstrumentationStack()->at(depth);
 }
 
@@ -309,7 +309,7 @@
           // While profiling, the return pc is restored from the side stack, except when walking
           // the stack for an exception where the side stack will be unwound in VisitFrame.
           if (GetQuickInstrumentationExitPc() == return_pc) {
-            instrumentation::InstrumentationStackFrame instrumentation_frame =
+            const instrumentation::InstrumentationStackFrame& instrumentation_frame =
                 GetInstrumentationStackFrame(instrumentation_stack_depth);
             instrumentation_stack_depth++;
             if (GetMethod() == Runtime::Current()->GetCalleeSaveMethod(Runtime::kSaveAll)) {