Add a manifest property declaring USB printers supported by an app.

extensions/common/api/_manifest_features.json

 // Copyright 2014 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.
 
 // This features file defines manifest keys implemented under src/extensions.
 // See extensions/common/features/* to understand this file, in particular
 // feature.h, simple_feature.h, and base_feature_provider.h.
 
 // To add a new whitelisted ID, SHA-1 it and force it to uppercase. In Bash:
 //
@@ skipping 251 matching lines @@
     "channel": "stable",
     "extension_types": [
       "extension", "platform_app", "legacy_packaged_app"
     ],
     "min_manifest_version": 2
   },
   "sockets": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   },
+  "usb_printers": {
+    "channel": "dev",
+    "extension_types": ["platform_app"]
+  },
   "version": {
     "channel": "stable",
     "extension_types": "all"
   },
   "version_name": {
     "channel": "stable",
     "extension_types": "all"
   },
   "web_accessible_resources": [
     {
       "channel": "stable",
       "extension_types": [
         "extension", "legacy_packaged_app", "hosted_app"
       ]
     },
     {
       "channel": "stable",
       "extension_types": ["platform_app"],
       "location": "component"
     }
   ],
   "webview": {
     "channel": "stable",
     "extension_types": ["platform_app"],
     "min_manifest_version": 2
   }
 }