Assert external strings in external string table. Remove TODOs.

src/heap-inl.h

Index: src/heap-inl.h
diff --git a/src/heap-inl.h b/src/heap-inl.h
index 6caa742f532b0503313846976383cf71de1aff40..9d0479781d7c00bdad2cb706355681b45ad5b513 100644
--- a/src/heap-inl.h
+++ b/src/heap-inl.h
@@ -718,13 +718,11 @@ void ExternalStringTable::Verify() {
 #ifdef DEBUG
   for (int i = 0; i < new_space_strings_.length(); ++i) {
     Object* obj = Object::cast(new_space_strings_[i]);
-    // TODO(yangguo): check that the object is indeed an external string.
     ASSERT(heap_->InNewSpace(obj));
     ASSERT(obj != HEAP->the_hole_value());
   }
   for (int i = 0; i < old_space_strings_.length(); ++i) {
     Object* obj = Object::cast(old_space_strings_[i]);
-    // TODO(yangguo): check that the object is indeed an external string.
     ASSERT(!heap_->InNewSpace(obj));
     ASSERT(obj != HEAP->the_hole_value());
   }