[unicode] Fix Utf8Decoder::is_one_byte to actually include latin1

Change-Id: Ibb3f6d5a0f2f24ff5359ca280f058a20c8215ac1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1624801
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61758}

src/strings/unicode-decoder.h

@@ -55,7 +55,7 @@ class V8_EXPORT_PRIVATE Utf8Decoder final {
   explicit Utf8Decoder(const Vector<const uint8_t>& chars);
 
   bool is_ascii() const { return encoding_ == Encoding::kAscii; }
-  bool is_one_byte() const { return encoding_ < Encoding::kLatin1; }
+  bool is_one_byte() const { return encoding_ <= Encoding::kLatin1; }
   int utf16_length() const { return utf16_length_; }
   int non_ascii_start() const { return non_ascii_start_; }