Add full TurboFan support for accessing SeqString contents.

src/compiler/access-builder.h

Index: src/compiler/access-builder.h
diff --git a/src/compiler/access-builder.h b/src/compiler/access-builder.h
index d6385e444dc87bc86d36157a92222fe6590622c7..a62482394bec946962cffb4c88e747beb50f47e6 100644
--- a/src/compiler/access-builder.h
+++ b/src/compiler/access-builder.h
@@ -53,6 +53,9 @@ class AccessBuilder FINAL : public AllStatic {
   static ElementAccess ForTypedArrayElement(ExternalArrayType type,
                                             bool is_external);
 
+  // Provides access to the charaters of sequential strings.
+  static ElementAccess ForSeqStringChar(String::Encoding encoding);
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(AccessBuilder);
 };