Add full TurboFan support for accessing SeqString contents.

src/runtime/runtime-strings.cc

Index: src/runtime/runtime-strings.cc
diff --git a/src/runtime/runtime-strings.cc b/src/runtime/runtime-strings.cc
index a4b2a9afb27050e73695cbad880ed4046b3acf2f..646e7be20f93efdbd216f0f2ff24c5d648fc1fb6 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -1265,6 +1265,15 @@ RUNTIME_FUNCTION(Runtime_StringCharAt) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_OneByteSeqStringGetChar) {
+  SealHandleScope shs(isolate);
+  DCHECK(args.length() == 2);
+  CONVERT_ARG_CHECKED(SeqOneByteString, string, 0);
+  CONVERT_INT32_ARG_CHECKED(index, 1);
+  return Smi::FromInt(string->SeqOneByteStringGet(index));
+}
+
+
 RUNTIME_FUNCTION(Runtime_OneByteSeqStringSetChar) {
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 3);
@@ -1276,6 +1285,15 @@ RUNTIME_FUNCTION(Runtime_OneByteSeqStringSetChar) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_TwoByteSeqStringGetChar) {
+  SealHandleScope shs(isolate);
+  DCHECK(args.length() == 2);
+  CONVERT_ARG_CHECKED(SeqTwoByteString, string, 0);
+  CONVERT_INT32_ARG_CHECKED(index, 1);
+  return Smi::FromInt(string->SeqTwoByteStringGet(index));
+}
+
+
 RUNTIME_FUNCTION(Runtime_TwoByteSeqStringSetChar) {
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 3);