Refactor PrinterDiscoverer and PrinterDetector to use a common interface.

chrome/browser/chromeos/printing/usb_printer_detector.cc

Index: chrome/browser/chromeos/printing/usb_printer_detector.cc
diff --git a/chrome/browser/chromeos/printing/usb_printer_detector.cc b/chrome/browser/chromeos/printing/usb_printer_detector.cc
index 5ce8fb61feb8ad64b396f37dc3d4b8a2251c3c68..3be8d3013999d71ace21998c2de21d068c48ada8 100644
--- a/chrome/browser/chromeos/printing/usb_printer_detector.cc
+++ b/chrome/browser/chromeos/printing/usb_printer_detector.cc
@@ -107,6 +107,15 @@ class UsbPrinterDetectorImpl : public UsbPrinterDetector,
     return GetPrintersLocked();
   }
 
+  void Start() override {
+    started_ = true;
+    observer_list_->Notify(FROM_HERE,
+                           &PrinterDetector::Observer::OnPrintersFound,
+                           GetPrintersLocked());
+    observer_list_->Notify(FROM_HERE,
+                           &PrinterDetector::Observer::OnPrinterScanComplete);
+  }
+
  private:
   std::vector<Printer> GetPrintersLocked() {
     pp_lock_.AssertAcquired();
@@ -143,13 +152,14 @@ class UsbPrinterDetectorImpl : public UsbPrinterDetector,
     base::AutoLock auto_lock(pp_lock_);
     if (base::ContainsKey(present_printers_, device->guid())) {
       present_printers_.erase(device->guid());
-      auto printers = GetPrintersLocked();
-      // We already have pp_lock_, so need to call the pre-locked version of
-      // GetPrinters to prevent deadlock.
-      observer_list_->Notify(
-          FROM_HERE,
-          &UsbPrinterDetector::Observer::OnAvailableUsbPrintersChanged,
-          GetPrintersLocked());
+      if (started_) {
+        auto printers = GetPrintersLocked();
+        // We already have pp_lock_, so need to call the pre-locked version of
+        // GetPrinters to prevent deadlock.
+        observer_list_->Notify(FROM_HERE,
+                               &PrinterDetector::Observer::OnPrintersFound,
+                               GetPrintersLocked());
+      }
     } else {
       // If the device has been removed but it's not in present_printers_, it
       // must still be in the setup flow.
@@ -274,12 +284,15 @@ class UsbPrinterDetectorImpl : public UsbPrinterDetector,
     } else {
       base::AutoLock auto_lock(pp_lock_);
       present_printers_.emplace(data->device->guid(), std::move(data->printer));
-      observer_list_->Notify(
-          FROM_HERE,
-          &UsbPrinterDetector::Observer::OnAvailableUsbPrintersChanged,
-          GetPrintersLocked());
+      if (started_) {
+        observer_list_->Notify(FROM_HERE,
+                               &PrinterDetector::Observer::OnPrintersFound,
+                               GetPrintersLocked());
+      }
     }
   }
+  // Has Start() been called yet?
+  bool started_ = false;
 
   // Map from USB GUID to Printer that we have detected as being currently
   // plugged in and have finished processing.  Note present_printers_ may be