[stubs] Port String.prototype.substr to TurboFan

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 16e77300a03ab6ba622b11a0d403dee8fa7bb42e..044592bb644c330a9ad495896c2aa712901d7dcc 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1404,6 +1404,8 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
                           Builtins::kStringPrototypeLocaleCompare, 1, true);
     SimpleInstallFunction(prototype, "normalize",
                           Builtins::kStringPrototypeNormalize, 0, false);
+    SimpleInstallFunction(prototype, "substr", Builtins::kStringPrototypeSubstr,
+                          2, true);
     SimpleInstallFunction(prototype, "substring",
                           Builtins::kStringPrototypeSubstring, 2, true);
     SimpleInstallFunction(prototype, "toString",