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

chrome/common/extensions/docs/templates/json/manifest.json

 {
   "app": {
     "documentation": "manifest/app",
     "example": {},
     "level": "required"
   },
   "app.background": {
     "example": {"scripts": ["background.js"]},
     "level": "required"
   },
@@ skipping 196 matching lines @@
     "example": {}
   },
   "update_url": {
     "documentation": "autoupdate",
     "example": "http://path/to/updateInfo.xml"
   },
   "url_handlers": {
     "documentation": "manifest/url_handlers",
     "example": {}
   },
+  "usb_printers": {
+    "documentation": "manifest/usb_printers",
+    "example": {
+      "filters": []
+    }
+  },
   "version": {
     "documentation": "manifest/version",
     "example": "versionString",
     "level": "required"
   },
   "version_name": {
     "documentation": "manifest/version",
     "example": "aString"
   },
   "web_accessible_resources": {
     "documentation": "manifest/web_accessible_resources",
     "example": []
   },
   "webview": {
     "documentation": "tags/webview#local_resources",
     "example": {}
   }
 }