[builtins] Migrate String.prototype.charCodeAt and String.prototype.charAt to TurboFan.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 0428f92830298e777630aa7ff87ae79268f70249..ba076a102d45d96069f7f3bfb72aa8d98c5910db 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1255,6 +1255,22 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
                                    attribs);
       string_map->AppendDescriptor(&d);
     }
+
+    // Create the %StringPrototype%
+    Handle<JSValue> prototype =
+        Handle<JSValue>::cast(factory->NewJSObject(string_fun, TENURED));
+    prototype->set_value(isolate->heap()->empty_string());
+    Accessors::FunctionSetPrototype(string_fun, prototype).Assert();
+
+    // Install the "constructor" property on the {prototype}.
+    JSObject::AddProperty(prototype, factory->constructor_string(), string_fun,
+                          DONT_ENUM);
+
+    // Install the String.prototype methods.
+    SimpleInstallFunction(prototype, "charAt", Builtins::kStringPrototypeCharAt,
+                          1, true);
+    SimpleInstallFunction(prototype, "charCodeAt",
+                          Builtins::kStringPrototypeCharCodeAt, 1, true);
   }
 
   {