[Contextual Search] Add Mojo-enabled API component.

components/contextual_search/common/contextual_search_js_api_service.mojom

+// Copyright 2015 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 contextual_search;
+
+// This service is implemented by the browser process and is used by the
+// renderer when a Contextual Search JavaScript API function is called.
+interface ContextualSearchJsApiService {
+
+  // Handle a call from the JS API to set the caption, and indicate whether
+  // the caption provides an answer (such as an actual definition), rather than
+  // just general notification of what kind of answer may be available.
+  HandleSetCaption(string message, bool does_answer);
+};