Add CHECKs to verify that we never finalize stale copies of external strings

BUG=none
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1160253010

Cr-Commit-Position: refs/heads/master@{#28811}

src/heap/mark-compact.cc

@@ -1679,13 +1679,20 @@ class StringTableCleaner : public ObjectVisitor {
       if (o->IsHeapObject() &&
           Marking::IsWhite(Marking::MarkBitFrom(HeapObject::cast(o)))) {
         if (finalize_external_strings) {
-          DCHECK(o->IsExternalString());
+          // We must never finalize a string that was copied (has a forwarding
+          // map).
+          CHECK(o->IsExternalString());
           heap_->FinalizeExternalString(String::cast(*p));
         } else {
           pointers_removed_++;
         }
         // Set the entry to the_hole_value (as deleted).
         *p = heap_->the_hole_value();
+      } else {
+        // Anything in the external string table has to be either a string or
+        // the hole.
+        CHECK(!finalize_external_strings || o->IsExternalString() ||
+              o->IsTheHole());
       }
     }
   }