ART: Add cause to GC wait-for-completion systrace

Bug: 122099093
Test: m
Change-Id: Ie777ba2c28200d6e54396ed7b450e7d2be833005
diff --git a/runtime/gc/heap.cc b/runtime/gc/heap.cc
index 8f9967f..8d19cd0 100644
--- a/runtime/gc/heap.cc
+++ b/runtime/gc/heap.cc
@@ -3531,7 +3531,7 @@
       running_collection_is_blocking_ = true;
       VLOG(gc) << "Waiting for a blocking GC " << cause;
     }
-    ScopedTrace trace("GC: Wait For Completion");
+    SCOPED_TRACE << "GC: Wait For Completion " << cause;
     // We must wait, change thread state then sleep on gc_complete_cond_;
     gc_complete_cond_->Wait(self);
     last_gc_type = last_gc_type_;