[string] Add missing two-to-one byte conversion

Use the "convert" parameter to SequentialStringKey to construct one-byte
strings out of two-byte input vectors, where appropriate.

Change-Id: I8a214b3960c677614d6f82ed3b29405e2e493e81
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2557981
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81062}

src/heap/factory-base.cc

@@ -645,6 +645,19 @@ Handle<SeqTwoByteString> FactoryBase<Impl>::NewTwoByteInternalizedString(
   return result;
 }
 
+template <typename Impl>
+Handle<SeqOneByteString>
+FactoryBase<Impl>::NewOneByteInternalizedStringFromTwoByte(
+    const base::Vector<const base::uc16>& str, uint32_t raw_hash_field) {
+  Handle<SeqOneByteString> result =
+      AllocateRawOneByteInternalizedString(str.length(), raw_hash_field);
+  DisallowGarbageCollection no_gc;
+  CopyChars(
+      result->GetChars(no_gc, SharedStringAccessGuardIfNeeded::NotNeeded()),
+      str.begin(), str.length());
+  return result;
+}
+
 template <typename Impl>
 template <typename SeqStringT>
 MaybeHandle<SeqStringT> FactoryBase<Impl>::NewRawStringWithMap(

src/heap/factory-base.h

@@ -213,6 +213,8 @@ class EXPORT_TEMPLATE_DECLARE(V8_EXPORT_PRIVATE) FactoryBase
       const base::Vector<const uint8_t>& str, uint32_t raw_hash_field);
   Handle<SeqTwoByteString> NewTwoByteInternalizedString(
       const base::Vector<const base::uc16>& str, uint32_t raw_hash_field);
+  Handle<SeqOneByteString> NewOneByteInternalizedStringFromTwoByte(
+      const base::Vector<const base::uc16>& str, uint32_t raw_hash_field);
 
   Handle<SeqOneByteString> AllocateRawOneByteInternalizedString(
       int length, uint32_t raw_hash_field);

src/objects/string-inl.h

@@ -359,6 +359,10 @@ class SequentialStringKey final : public StringTableKey {
     if (sizeof(Char) == 1) {
       internalized_string_ = isolate->factory()->NewOneByteInternalizedString(
           base::Vector<const uint8_t>::cast(chars_), raw_hash_field());
+    } else if (convert_) {
+      internalized_string_ =
+          isolate->factory()->NewOneByteInternalizedStringFromTwoByte(
+              base::Vector<const uint16_t>::cast(chars_), raw_hash_field());
     } else {
       internalized_string_ = isolate->factory()->NewTwoByteInternalizedString(
           base::Vector<const uint16_t>::cast(chars_), raw_hash_field());