Make CUPS USB printing play better with the settings page. This change does several things:

chrome/browser/chromeos/printer_detector/printer_detector.h

Index: chrome/browser/chromeos/printer_detector/printer_detector.h
diff --git a/chrome/browser/chromeos/printer_detector/printer_detector.h b/chrome/browser/chromeos/printer_detector/printer_detector.h
index e7a5b3d5828d89b36b25bbf817639688f68f11d0..1789377df473c48e3d70b2087bfaa11f7f732854 100644
--- a/chrome/browser/chromeos/printer_detector/printer_detector.h
+++ b/chrome/browser/chromeos/printer_detector/printer_detector.h
@@ -7,8 +7,10 @@
 
 #include <memory>
 #include <string>
+#include <vector>
 
 #include "base/macros.h"
+#include "chromeos/printing/printer_configuration.h"
 #include "components/keyed_service/core/keyed_service.h"
 
 class NotificationUIManager;
@@ -34,6 +36,15 @@ namespace chromeos {
 // CUPS backend.
 class PrinterDetector : public KeyedService {
  public:
+  class Observer {
+   public:
+    virtual ~Observer() = default;
+
+    // The set of available printers has changed.
+    virtual void OnAvailableUsbPrintersChanged(
+        const std::vector<Printer>& printers) = 0;
+  };
+
   // Factory function for the Legacy implementation.
   static std::unique_ptr<PrinterDetector> CreateLegacy(Profile* profile);
 
@@ -41,6 +52,16 @@ class PrinterDetector : public KeyedService {
   static std::unique_ptr<PrinterDetector> CreateCups(Profile* profile);
   ~PrinterDetector() override {}
 
+  // Observer management.  Note these are only implemented for the cups backend.
+  // TODO(justincarlson) - Change these all to pure virtual functions when the
+  // legacy backend is retired.
+
+  virtual void AddObserver(Observer* observer) {}
+  virtual void RemoveObserver(Observer* observer) {}
+
+  // Get the current set of detected printers.
+  virtual std::vector<Printer> GetPrinters();
+
  protected:
   PrinterDetector() = default;