Add support for changing roots through the root visitor callback.

Needed for copying collectors.

Change-Id: Icc4a342a57e0cfb79587edb02ef8c85e08808877
diff --git a/runtime/sirt_ref.h b/runtime/sirt_ref.h
index 81f0dff..25d6fb3 100644
--- a/runtime/sirt_ref.h
+++ b/runtime/sirt_ref.h
@@ -30,7 +30,7 @@
     self_->PushSirt(&sirt_);
   }
   ~SirtRef() {
-    CHECK(self_->PopSirt() == &sirt_);
+    CHECK_EQ(self_->PopSirt(), &sirt_);
   }
 
   T& operator*() const { return *get(); }