Flatten strings before internalization

BUG=chromium:592707
LOG=n

Review URL: https://codereview.chromium.org/1775913002

Cr-Commit-Position: refs/heads/master@{#34584}

src/objects.cc

@@ -17087,7 +17087,7 @@ bool SeqOneByteSubStringKey::IsMatch(Object* string) {
 class InternalizedStringKey : public HashTableKey {
  public:
   explicit InternalizedStringKey(Handle<String> string)
-      : string_(string) { }
+      : string_(String::Flatten(string)) {}
 
   bool IsMatch(Object* string) override {
     return String::cast(string)->Equals(*string_);
@@ -17949,7 +17949,7 @@ void StringTable::EnsureCapacityForDeserialization(Isolate* isolate,
                                                    int expected) {
   Handle<StringTable> table = isolate->factory()->string_table();
   // We need a key instance for the virtual hash function.
-  InternalizedStringKey dummy_key(Handle<String>::null());
+  InternalizedStringKey dummy_key(isolate->factory()->empty_string());
   table = StringTable::EnsureCapacity(table, expected, &dummy_key);
   isolate->heap()->SetRootStringTable(*table);
 }