Add GC cause to WaitForGcToComplete message.

Change-Id: I8fe107d90a84de065c407b8d29fd106267ac440d
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index fbc0460..027feee 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -160,7 +160,7 @@
   Trace::Shutdown();
 
   // Make sure to let the GC complete if it is running.
-  heap_->WaitForGcToComplete(self);
+  heap_->WaitForGcToComplete(gc::kGcCauseBackground, self);
   heap_->DeleteThreadPool();
 
   // Make sure our internal threads are dead before we start tearing down things they're using.