[Contextual Search] Add Mojo-enabled API component.

components/contextual_search/contextual_search_api_service.mojom

Index: components/contextual_search/contextual_search_api_service.mojom
diff --git a/components/contextual_search/contextual_search_api_service.mojom b/components/contextual_search/contextual_search_api_service.mojom
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+// 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 ContextualSearchApiService {
+
+  // Handle the "setCaption" function.
+  HandleSetCaption(string message);
+};