Don't DCHECK that the weak closure wasn't yet overapproximated

Maybe it was overapproximated via the idle notification already, so just
don't retry in that case

BUG=chromium:469023
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1025603003

Cr-Commit-Position: refs/heads/master@{#27335}

src/heap/heap.cc

@@ -736,8 +736,9 @@ void Heap::HandleGCRequest() {
     return;
   }
   DCHECK(FLAG_overapproximate_weak_closure);
-  DCHECK(!incremental_marking()->weak_closure_was_overapproximated());
-  OverApproximateWeakClosure("GC interrupt");
+  if (!incremental_marking()->weak_closure_was_overapproximated()) {
+    OverApproximateWeakClosure("GC interrupt");
+  }
 }