Correct property descriptors on GeneratorPrototype

The ES6 specification does not explicitly state the attributes for the
'next' and 'throw' property descriptors, so their values are defined by
Section 17 [1]:

> Every other data property described in clauses 18 through 26 and in
> Annex B.2 has the attributes
> { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }
> unless otherwise specified.

[1]
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-ecmascript-standard-built-in-objects

BUG=v8:3986
LOG=N
R=wingo,arv

Review URL: https://codereview.chromium.org/1051363003

Cr-Commit-Position: refs/heads/master@{#27770}

src/generator.js

@@ -89,7 +89,7 @@ function SetUpGenerators() {
   // Set up non-enumerable functions on the generator prototype object.
   var GeneratorObjectPrototype = GeneratorFunctionPrototype.prototype;
   InstallFunctions(GeneratorObjectPrototype,
-                   DONT_ENUM | DONT_DELETE | READ_ONLY,
+                   DONT_ENUM,
                    ["next", GeneratorObjectNext,
                     "throw", GeneratorObjectThrow]);
 

test/mjsunit/es6/generators-runtime.js

@@ -114,7 +114,28 @@ function TestGeneratorObjectPrototype() {
 
   assertArrayEquals(expected_property_names, found_property_names);
 
-  iterator_desc = Object.getOwnPropertyDescriptor(GeneratorObjectPrototype,
+  var constructor_desc = Object.getOwnPropertyDescriptor(
+      GeneratorObjectPrototype, "constructor");
+  assertTrue(constructor_desc !== undefined);
+  assertFalse(constructor_desc.writable);
+  assertFalse(constructor_desc.enumerable);
+  assertTrue(constructor_desc.configurable);
+
+  var next_desc = Object.getOwnPropertyDescriptor(GeneratorObjectPrototype,
+      "next");
+  assertTrue(next_desc !== undefined);
+  assertTrue(next_desc.writable);
+  assertFalse(next_desc.enumerable);
+  assertTrue(next_desc.configurable);
+
+  var throw_desc = Object.getOwnPropertyDescriptor(GeneratorObjectPrototype,
+      "throw");
+  assertTrue(next_desc !== undefined);
+  assertTrue(next_desc.writable);
+  assertFalse(next_desc.enumerable);
+  assertTrue(next_desc.configurable);
+
+  var iterator_desc = Object.getOwnPropertyDescriptor(GeneratorObjectPrototype,
       Symbol.iterator);
   assertTrue(iterator_desc !== undefined);
   assertFalse(iterator_desc.writable);