[heap] Report proper OOM failure if semi-space copy fails.

src/heap/scavenger.cc

Index: src/heap/scavenger.cc
diff --git a/src/heap/scavenger.cc b/src/heap/scavenger.cc
index 3e9f67b896ee614ce5163dfa2be995b2de557a31..31f1ee55b73dcdbc57007881dfa63d951669c4c7 100644
--- a/src/heap/scavenger.cc
+++ b/src/heap/scavenger.cc
@@ -236,7 +236,7 @@ class ScavengingVisitor : public StaticVisitorBase {
     // If promotion failed, we try to copy the object to the other semi-space
     if (SemiSpaceCopyObject<alignment>(map, slot, object, object_size)) return;
 
-    UNREACHABLE();
+    FatalProcessOutOfMemory("Scavenger: semi-space copy\n");
   }