Fix crash when .mhtml/.pdf files are opened from download shelf on Chrome OS.

chrome/browser/chromeos/file_manager/file_browser_handlers.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/browser/chromeos/file_manager/file_browser_handlers.h"
 
 #include "base/bind.h"
 #include "base/file_util.h"
 #include "base/i18n/case_conversion.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 462 matching lines @@
     const std::vector<FileSystemURL>& file_urls,
     const file_tasks::FileTaskFinishedCallback& done) {
   // Forbid calling undeclared handlers.
   if (!FindFileBrowserHandlerForActionId(extension, action_id))
     return false;
 
   // Some action IDs of the file manager's file browser handlers require the
   // files to be directly opened with the browser.
   if (ShouldBeOpenedWithBrowser(extension->id(), action_id)) {
     const bool result = OpenFilesWithBrowser(profile, file_urls);
-    done.Run(result
-                 ? extensions::api::file_browser_private::TASK_RESULT_OPENED
-                 : extensions::api::file_browser_private::TASK_RESULT_FAILED);
+    if (!done.is_null()) {
+      done.Run(result
+                   ? extensions::api::file_browser_private::TASK_RESULT_OPENED
+                   : extensions::api::file_browser_private::TASK_RESULT_FAILED);
+    }
     return result;
   }
 
   // The executor object will be self deleted on completion.
   (new FileBrowserHandlerExecutor(
       profile, extension, action_id))->Execute(file_urls, done);
   return true;
 }
 
 bool IsFallbackFileBrowserHandler(const file_tasks::TaskDescriptor& task) {
@@ skipping 56 matching lines @@
       common_handlers.erase(gallery_iter);
     else
       common_handlers.erase(watch_iter);
   }
 
   return common_handlers;
 }
 
 }  // namespace file_browser_handlers
 }  // namespace file_manager