Use |Printer::protocol| instead of manually checking for scheme

Change-Id: I3fd7213c4ff62b17e6d2f101c35920e426f09763
Reviewed-on: https://chromium-review.googlesource.com/c/1448415
Commit-Queue: Vladislav Kuzkokov <vkuzkokov@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628321}

chrome/browser/chromeos/printing/cups_printers_manager.cc

@@ -80,7 +80,7 @@ void FilterOutPrinters(std::vector<Printer>* printers,
 
 // Return true if this is a USB printer.
 bool IsUsbPrinter(const Printer& printer) {
-  return base::StringPiece(printer.uri()).starts_with("usb://");
+  return printer.GetProtocol() == Printer::kUsb;
 }
 
 class CupsPrintersManagerImpl : public CupsPrintersManager,