Expose the "automatic downloads" content setting to extensions.

chrome/common/extensions/api/content_settings.json

 // 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.
 
 [
   {
     "namespace": "contentSettings",
     "description": "Use the <code>chrome.contentSettings</code> API to change settings that control whether websites can use features such as cookies, JavaScript, and plug-ins. More generally speaking, content settings allow you to customize Chrome's behavior on a per-site basis instead of globally.",
     "compiler_options": {
       "generate_type_functions": true
@@ skipping 250 matching lines @@
           {"type":"string", "enum": ["allow", "block", "ask"]}
         ]
       },
       "unsandboxedPlugins": {
         "$ref": "ContentSetting",
         "description": "Whether to allow sites to run plug-ins unsandboxed. One of <br><var>allow</var>: Allow sites to run plug-ins unsandboxed,<br><var>block</var>: Don't allow sites to run plug-ins unsandboxed,<br><var>ask</var>: Ask when a site wants to run a plug-in unsandboxed. <br>Default is <var>ask</var>.<br>The primary URL is the URL of the top-level frame. The secondary URL is not used.",
         "value": [
           "ppapi-broker",
           {"type":"string", "enum": ["allow", "block", "ask"]}
         ]
+      },
+      "automaticDownloads": {
+        "$ref": "ContentSetting",
+        "description": "Whether to allow sites to download multiple files automatically. One of <br><var>allow</var>: Allow sites to download multiple files automatically,<br><var>block</var>: Don't allow sites to download multiple files automatically,<br><var>ask</var>: Ask when a site wants to download files automatically after the first file. <br>Default is <var>ask</var>.<br>The primary URL is the URL of the top-level frame. The secondary URL is not used.",

[Mike West, 2015/02/03 20:21:53]

Does 'block' just block downloading multiple files automatically? What's the
behavior for the first file?

+        "value": [
+          "multiple-automatic-downloads",
+          {"type":"string", "enum": ["allow", "block", "ask"]}
+        ]
       }
     }
   }
 ]