Assert external strings in external string table. Remove TODOs.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index ad352bce50ee38990042edfe746708872799c73d..57c3d70d3c06555b75c76b1ff06b56a060586a43 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -7886,6 +7886,7 @@ void ExternalStringTable::CleanUp() {
     if (new_space_strings_[i] == heap_->the_hole_value()) {
       continue;
     }
+    ASSERT(new_space_strings_[i]->IsExternalString());
     if (heap_->InNewSpace(new_space_strings_[i])) {
       new_space_strings_[last++] = new_space_strings_[i];
     } else {
@@ -7900,6 +7901,7 @@ void ExternalStringTable::CleanUp() {
     if (old_space_strings_[i] == heap_->the_hole_value()) {
       continue;
     }
+    ASSERT(old_space_strings_[i]->IsExternalString());
     ASSERT(!heap_->InNewSpace(old_space_strings_[i]));
     old_space_strings_[last++] = old_space_strings_[i];
   }