[Fuchsia] Define BindingsManager for sourcing bindings from CastAgent.

Defines the FIDL service which will be provided by the CastAgent to
supply the CastRunner with bindings scripts and receive channels
connected to the scripts.


Bug: 953958
Change-Id: Ie1962e1395975b5daf3e3a0f13140f79676adfde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592854
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657374}

fuchsia/BUILD.gn

@@ -15,6 +15,7 @@ fidl_library("cast_fidl") {
   namespace = "chromium"
 
   sources = [
+    "fidl/cast/api_bindings.fidl",
     "fidl/cast/application_config.fidl",
     "fidl/cast/cast_channel.fidl",
     "fidl/cast/queryable_data.fidl",

fuchsia/fidl/cast/api_bindings.fidl

+// Copyright 2019 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.
+
+library chromium.cast;
+
+using fuchsia.mem;
+using fuchsia.web;
+
+/// Provides Cast platform API binding scripts and methods for connecting those
+/// scripts with Agent backends.
+[Discoverable]
+protocol ApiBindings {
+    /// Gets the list of bindings to early-inject into the page at load time.
+    GetAll() -> (vector<ApiBinding> bindings);
+
+    /// Should be invoked when a connecting a named MessagePort to a native
+    /// bindings backend.
+    Connect(string api_name, fuchsia.web.MessagePort message_port);
+};
+
+table ApiBinding {
+    /// Script to execute before the load of a web document.
+    1: fuchsia.mem.Buffer before_load_script;
+};