Add full TurboFan support for accessing SeqString contents.

src/runtime/runtime.h

Index: src/runtime/runtime.h
diff --git a/src/runtime/runtime.h b/src/runtime/runtime.h
index a5cefd67a8a333f7809f14862d4962f75bdf00ca..bae9945f697fe4bec851234b83b7e28ecffbf5b5 100644
--- a/src/runtime/runtime.h
+++ b/src/runtime/runtime.h
@@ -647,7 +647,9 @@ namespace internal {
   F(DateField, 2 /* date object, field index */, 1)         \
   F(StringCharFromCode, 1, 1)                               \
   F(StringCharAt, 2, 1)                                     \
+  F(OneByteSeqStringGetChar, 2, 1)                          \
   F(OneByteSeqStringSetChar, 3, 1)                          \
+  F(TwoByteSeqStringGetChar, 2, 1)                          \
   F(TwoByteSeqStringSetChar, 3, 1)                          \
   F(ObjectEquals, 2, 1)                                     \
   F(IsObject, 1, 1)                                         \