[Media Router] Consistently refer to issue's learn more help page by ID.

chrome/browser/media/router/mojo/media_router.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 media_router.interfaces;
 
 // Represents an output sink to which media can be routed.
 struct MediaSink {
   enum IconType {
     CAST,
@@ skipping 79 matching lines @@
   string title;
 
   // Message about issue detail or how to handle issue.
   // Messages should be suitable for end users to decide which actions to take.
   string? message;
 
   ActionType default_action;
 
   array<ActionType>? secondary_actions;
 
-  // A help page to be opened if users select learn_more.
-  string? help_url;
+  // The ID of the help page to be opened if users select learn_more.
+  int32 help_page_id;
 };
 
 struct RouteMessage {
   enum Type {
     TEXT,
     BINARY
   };
   // The type of this message.
   Type type;
   // Used when the |type| is TEXT.
@@ skipping 257 matching lines @@
   // changed to |state|.
   OnPresentationConnectionStateChanged(
       string route_id, PresentationConnectionState state);
 
   // Called when the presentation connected to route |route_id| has closed.
   OnPresentationConnectionClosed(
       string route_id, PresentationConnectionCloseReason reason,
       string message);
 };