Add mojo service for CopylessPaste in Blink

third_party/WebKit/public/platform/modules/document_metadata/copyless_paste.mojom

+// Copyright 2017 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.
+
+module blink.mojom;
+
+union Values {
+  array<bool> bool_values;
+  array<int64> long_values;
+  array<string> string_values;
+  array<Entity> entity_values;
+};
+
+struct Property {
+  string name;
+  Values values;
+};
+
+struct Entity {
+  string type;
+  array<Property> properties;
+};
+
+struct WebPage {
+  string url;
+  string title;
+  array<Entity> entities;
+};
+
+interface CopylessPaste {
+  GetEntities() => (WebPage page);
+};