[heap] Minor MC: Flush concurrent external memory counter

BUG=chromium:651354

Review-Url: https://codereview.chromium.org/2871543002
Cr-Commit-Position: refs/heads/master@{#45159}

src/heap/mark-compact.cc

@@ -2610,6 +2610,8 @@ void MinorMarkCompactCollector::CollectGarbage() {
       marking_state(p).ClearLiveness();
     }
   }
+
+  heap()->account_external_memory_concurrently_freed();
 }
 
 void MinorMarkCompactCollector::ClearNonLiveReferences() {