v8binding: Changes @@unscopables's prototype to ES null.

Implements step 8.1. of
https://heycam.github.io/webidl/#create-an-interface-prototype-object

Bug: 811029
Change-Id: I0feb1c16cf12585db8847c82d4efa342701bebcb
Reviewed-on: https://chromium-review.googlesource.com/1098914
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567580}

third_party/WebKit/LayoutTests/external/wpt/WebIDL/ecmascript-binding/interface-prototype-object-expected.txt

 This is a testharness.js-based test.
 FAIL The class string of an interface prototype object is the concatenation of the interface's identifier and the string 'Prototype'. assert_equals: expected "[object DocumentPrototype]" but got "[object Document]"
+PASS [Unscopable] extended attribute makes @@unscopables object on the prototype object, whose prototype is null.
 Harness: the test ran to completion.
 

third_party/WebKit/LayoutTests/external/wpt/WebIDL/ecmascript-binding/interface-prototype-object.html

@@ -16,4 +16,12 @@ test(function() {
   assert_equals(Document.prototype[Symbol.toStringTag], "DocumentPrototype");
 }, "The class string of an interface prototype object is the concatenation of " +
    "the interface's identifier and the string 'Prototype'.");
+
+test(function() {
+  // https://heycam.github.io/webidl/#create-an-interface-prototype-object
+  assert_own_property(Element.prototype, Symbol.unscopables, "Element.prototype has @@unscopables.");
+  let unscopables = Element.prototype[Symbol.unscopables];
+  assert_equals(typeof unscopables, "object", "@@unscopables is an Object.");
+  assert_equals(Object.getPrototypeOf(unscopables), null, "@@unscopables's prototype is null.");
+}, "[Unscopable] extended attribute makes @@unscopables object on the prototype object, whose prototype is null.");
 </script>

third_party/blink/renderer/bindings/templates/interface_base.cpp.tmpl

@@ -985,7 +985,12 @@ void {{v8_class_or_partial}}::InstallConditionalFeatures(
     if (prototypeObject->HasOwnProperty(context, unscopablesSymbol).To(&has_unscopables) && has_unscopables) {
       unscopables = prototypeObject->Get(context, unscopablesSymbol).ToLocalChecked().As<v8::Object>();
     } else {
+      // Web IDL 3.6.3. Interface prototype object
+      // https://heycam.github.io/webidl/#create-an-interface-prototype-object
+      // step 8.1. Let unscopableObject be the result of performing
+      //   ! ObjectCreate(null).
       unscopables = v8::Object::New(isolate);
+      unscopables->SetPrototype(context, v8::Null(isolate)).ToChecked();
     }
     {% for name, runtime_enabled_feature_name in unscopables %}
     {% filter runtime_enabled(runtime_enabled_feature_name) %}

third_party/blink/renderer/bindings/tests/results/core/v8_test_object.cc

@@ -13828,7 +13828,12 @@ void V8TestObject::InstallConditionalFeatures(
     if (prototypeObject->HasOwnProperty(context, unscopablesSymbol).To(&has_unscopables) && has_unscopables) {
       unscopables = prototypeObject->Get(context, unscopablesSymbol).ToLocalChecked().As<v8::Object>();
     } else {
+      // Web IDL 3.6.3. Interface prototype object
+      // https://heycam.github.io/webidl/#create-an-interface-prototype-object
+      // step 8.1. Let unscopableObject be the result of performing
+      //   ! ObjectCreate(null).
       unscopables = v8::Object::New(isolate);
+      unscopables->SetPrototype(context, v8::Null(isolate)).ToChecked();
     }
     unscopables->CreateDataProperty(
         context, V8AtomicString(isolate, "unscopableLongAttribute"), v8::True(isolate))

third_party/blink/renderer/bindings/tests/results/modules/v8_test_interface_partial.cc

@@ -568,7 +568,12 @@ void V8TestInterfacePartial::InstallConditionalFeatures(
     if (prototypeObject->HasOwnProperty(context, unscopablesSymbol).To(&has_unscopables) && has_unscopables) {
       unscopables = prototypeObject->Get(context, unscopablesSymbol).ToLocalChecked().As<v8::Object>();
     } else {
+      // Web IDL 3.6.3. Interface prototype object
+      // https://heycam.github.io/webidl/#create-an-interface-prototype-object
+      // step 8.1. Let unscopableObject be the result of performing
+      //   ! ObjectCreate(null).
       unscopables = v8::Object::New(isolate);
+      unscopables->SetPrototype(context, v8::Null(isolate)).ToChecked();
     }
     unscopables->CreateDataProperty(
         context, V8AtomicString(isolate, "unscopableVoidMethod"), v8::True(isolate))