[regexp] Fix StringCharacterPosition for ThinStrings

src/regexp/regexp-macro-assembler.cc

Index: src/regexp/regexp-macro-assembler.cc
diff --git a/src/regexp/regexp-macro-assembler.cc b/src/regexp/regexp-macro-assembler.cc
index 73797e4416b8b198e3c13df50b56587bd6c04fa3..2e3a8a2f76877eb90bac9dd699ae03ba1e86a505 100644
--- a/src/regexp/regexp-macro-assembler.cc
+++ b/src/regexp/regexp-macro-assembler.cc
@@ -133,7 +133,8 @@ const byte* NativeRegExpMacroAssembler::StringCharacterPosition(
   } else if (subject->IsSlicedString()) {
     start_index += SlicedString::cast(subject)->offset();
     subject = SlicedString::cast(subject)->parent();
-  } else if (subject->IsThinString()) {
+  }
+  if (subject->IsThinString()) {
     subject = ThinString::cast(subject)->actual();
   }
   DCHECK(start_index >= 0);
@@ -148,6 +149,7 @@ const byte* NativeRegExpMacroAssembler::StringCharacterPosition(
     return reinterpret_cast<const byte*>(
         ExternalOneByteString::cast(subject)->GetChars() + start_index);
   } else {
+    DCHECK(subject->IsExternalTwoByteString());
     return reinterpret_cast<const byte*>(
         ExternalTwoByteString::cast(subject)->GetChars() + start_index);
   }