Add Origin Trials Test interface, rather than attaching trials directly to internals.

third_party/WebKit/Source/core/testing/OriginTrialsTest.idl

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+interface OriginTrialsTest {
+    [OriginTrialEnabled=OriginTrialsSampleAPI] readonly attribute boolean normalAttribute;
+    [OriginTrialEnabled=OriginTrialsSampleAPI] static readonly attribute boolean staticAttribute;
+    [OriginTrialEnabled=OriginTrialsSampleAPI] const unsigned short CONSTANT = 1;
+
+    // This attribute uses native code to test whether the trial is enabled,
+    // and throws an exception in JavaScript if it is not.
+    [CallWith=ScriptState, RaisesException] readonly attribute boolean throwingAttribute;
+
+    // This attribute is available whether or not the trial is enabled.
+    readonly attribute boolean unconditionalAttribute;
+
+    // This attribute mirrors |unconditionalAttribute| above, but should only be
+    // available if the trial is enabled.
+    [OriginTrialEnabled=OriginTrialsSampleAPI, ImplementedAs=unconditionalAttribute] readonly attribute boolean bindingsTest;
+};