Update UI for extension destinations in print preview destination list

extensions/browser/api/printer_provider/printer_provider_api.h

 // Copyright 2015 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.
 
 #ifndef EXTENSIONS_BROWSER_API_PRINTER_PROVIDER_PRINTER_PROVIDER_API_H_
 #define EXTENSIONS_BROWSER_API_PRINTER_PROVIDER_PRINTER_PROVIDER_API_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 29 matching lines @@
   // Returns generic error string for print request.
   static std::string GetDefaultPrintError();
 
   // The API currently cannot handle very large files, so the document size that
   // may be sent to an extension is restricted.
   // TODO(tbarzic): Fix the API to support huge documents.
   static const int kMaxDocumentSize = 50 * 1000 * 1000;
 
   ~PrinterProviderAPI() override {}
 
+  // Requests list of supported printers from extensions implementing
+  // chrome.printerProvider API. It dispatches
+  // chrome.printerProvider.onGetPrintersRequested event. The callback is
+  // called once for every extension handling the event with a list of its
+  // supported printers. The printer values reported by an extension are
+  // added "extensionId" property that is set to the ID of the extension
+  // returning the list and "extensionName" property set to the extension's
+  // name.
+  // Note that the "id" property of printer values reported by an extension are
+  // rewriten as "<extension_id>:<id>" to ensure they are unique across
+  // different extensions.
   virtual void DispatchGetPrintersRequested(
       const GetPrintersCallback& callback) = 0;
 
   // Requests printer capability for a printer with id |printer_id|.
   // |printer_id| should be one of the printer ids reported by |GetPrinters|
   // callback.
   // It dispatches chrome.printerProvider.onGetCapabilityRequested event
   // to the extension that manages the printer (which can be determined from
   // |printer_id| value).
   // |callback| is passed a dictionary value containing printer capabilities as
   // reported by the extension.
   virtual void DispatchGetCapabilityRequested(
       const std::string& printer_id,
       const GetCapabilityCallback& callback) = 0;
 
   // It dispatches chrome.printerProvider.onPrintRequested event with the
   // provided print job. The event is dispatched only to the extension that
   // manages printer with id |job.printer_id|.
   // |callback| is passed the print status returned by the extension, and it
   // must not be null.
   virtual void DispatchPrintRequested(const PrinterProviderPrintJob& job,
                                       const PrintCallback& callback) = 0;
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_BROWSER_API_PRINTER_PROVIDER_PRINTER_PROVIDER_API_H_