[api] Add a switch that controls if ES2015 tail call elimination feature is enabled or not.

include/v8-debug.h

Index: include/v8-debug.h
diff --git a/include/v8-debug.h b/include/v8-debug.h
index 0d0ee739c0ed39b44e566cd490a2315ba73eb849..3c973835ad4d59e1d1082d39c0a4810045a42f5e 100644
--- a/include/v8-debug.h
+++ b/include/v8-debug.h
@@ -276,6 +276,12 @@ class V8_EXPORT Debug {
    */
   static MaybeLocal<Array> GetInternalProperties(Isolate* isolate,
                                                  Local<Value> value);
+
+  /**
+   * Defines if the ES2015 tail call elimination feature is enabled or not.
+   */
+  static bool IsTailCallEliminationEnabled(Isolate* isolate);
+  static void SetTailCallEliminationEnabled(Isolate* isolate, bool enabled);

[jochen (gone - plz use gerrit), 2016/03/29 15:12:15]

Can you add a comment saying whether this deopts all existing code?

In the future, it might be the case that you can't disable TCO, right? maybe
SetTail... should return a bool indicating whether or not it was indeed
disabled?

or should we just automatically disable TCO if the debugger is engaged?

[Igor Sheludko, 2016/03/29 15:32:19]

1) Probably when we decide to avoid disabling TCO we should remove these
functions from the API.
2) We decided to separate "TCO enabled" from "Debugger is enabled" in order to
let the user debug his code the same way it is run in without debugger. But if
user wants to see less confusing picture it could disable TCO in DevTools.

 };