arc: Make IntentHelper::OpenFileToRead's return value nullable

components/arc/common/intent_helper.mojom

 // Copyright 2016 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.
 //
 // Next MinVersion: 16
 
 module arc.mojom;
 
 import "scale_factor.mojom";
 
@@ skipping 118 matching lines @@
   // Handles the list of URLs by sending a specified intent to the handler.
   [MinVersion=5] HandleUrlList@7(array<UrlWithMimeType> urls,
                                  ActivityName activity,
                                  ActionType action_type);
 
   // Establishes full-duplex communication with the host.
   Init@0(IntentHelperHost host_ptr);
 
   // Asks the ContentResolver to get an FD to read the file specified by the
   // URL.
-  [MinVersion=15] OpenFileToRead@12(string url) => (handle fd);
+  [MinVersion=15] OpenFileToRead@12(string url) => (handle? fd);
 
   // Requests 48dp * 48dp icons of the |activities| suitable for the
   // |scale_factor|. An array of icon data will be returned.
   [MinVersion=3] RequestActivityIcons@4(array<ActivityName> activities,
                                         ScaleFactor scale_factor)
       => (array<ActivityIcon> icons);
 
   // Requests a list of packages that can handle |intent|.
   [MinVersion=12] RequestIntentHandlerList@9(IntentInfo intent)
       => (array<IntentHandlerInfo> handlers);
@@ skipping 10 matching lines @@
   // specified.  Data can be sent as extras by including a JSON map string which
   // will be automatically converted to a bundle accessible by the receiver.
   //
   // Note: Broadcasts can only be sent to whitelisted packages.  Packages can be
   // added to the whitelist in ArcBridgeService.java in the Android source.
   [MinVersion=1] SendBroadcast@1(string action,
                                  string package_name,
                                  string cls,
                                  string extras);
 };