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

extensions/common/manifest_constants.cc

Index: extensions/common/manifest_constants.cc
diff --git a/extensions/common/manifest_constants.cc b/extensions/common/manifest_constants.cc
index 528307bf11c7c80718115396797cbad6bb7201ff..534cc5b53484da1d0df48f6d03ae27d61eac24e5 100644
--- a/extensions/common/manifest_constants.cc
+++ b/extensions/common/manifest_constants.cc
@@ -174,6 +174,7 @@ const char kUIOverride[] = "chrome_ui_overrides";
 const char kUpdateURL[] = "update_url";
 const char kUrlHandlers[] = "url_handlers";
 const char kUrlHandlerTitle[] = "title";
+const char kUsbPrinters[] = "usb_printers";
 const char kVersion[] = "version";
 const char kVersionName[] = "version_name";
 const char kWebAccessibleResources[] = "web_accessible_resources";