Turn sliced strings into flat cons-strings upon internalization

Flat cons-strings point directly to the internalized version of the string. This makes reinternalization much faster.

BUG=

Review-Url: https://codereview.chromium.org/1932663002
Cr-Commit-Position: refs/heads/master@{#35852}

src/objects.cc

@@ -17232,6 +17232,16 @@ Handle<String> StringTable::LookupString(Isolate* isolate,
     Handle<ConsString> cons = Handle<ConsString>::cast(string);
     cons->set_first(*result);
     cons->set_second(isolate->heap()->empty_string());
+  } else if (string->IsSlicedString()) {
+    STATIC_ASSERT(ConsString::kSize == SlicedString::kSize);
+    DisallowHeapAllocation no_gc;
+    bool one_byte = result->IsOneByteRepresentation();
+    Handle<Map> map = one_byte ? isolate->factory()->cons_one_byte_string_map()
+                               : isolate->factory()->cons_string_map();
+    string->set_map(*map);
+    Handle<ConsString> cons = Handle<ConsString>::cast(string);
+    cons->set_first(*result);
+    cons->set_second(isolate->heap()->empty_string());
   }
   return result;
 }