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

chrome/browser/chromeos/printing/printer_discoverer.h

Index: chrome/browser/chromeos/printing/printer_discoverer.h
diff --git a/chrome/browser/chromeos/printing/printer_discoverer.h b/chrome/browser/chromeos/printing/printer_discoverer.h
index b0a863900872896998b4bbf422fb102b250a7fc3..20fd8bc9b5bf9096e3489c249a4e506dad41f9df 100644
--- a/chrome/browser/chromeos/printing/printer_discoverer.h
+++ b/chrome/browser/chromeos/printing/printer_discoverer.h
@@ -10,50 +10,45 @@
 
 #include "chromeos/printing/printer_configuration.h"
 
+class Profile;
+
 namespace chromeos {
 
 // Interface for printer discovery.  Constructs Printer objects from USB and
-// zeroconf (DNS-SD) printers.
+// zeroconf (DNS-SD) printers.  All functions in this class must be called
+// from a sequenced context.
 class CHROMEOS_EXPORT PrinterDiscoverer {
  public:
   // Interface for objects interested in detected printers.
   class Observer {
    public:
-    // Called after discovery has started.
-    virtual void OnDiscoveryStarted() {}
-
-    // Called when discovery is stopping.  OnPrintersFound will not be
-    // called after this occurs.
-    virtual void OnDiscoveryStopping() {}
-
-    // Called after all printers have been discovered and the observers
-    // notified.
-    virtual void OnDiscoveryDone() {}
-
-    // Called with a collection of printers as they are discovered.  Printer
-    // objects must be copied if they are retained outside of the scope of this
-    // function.
-    virtual void OnPrintersFound(const std::vector<Printer>& printers) {}
+    virtual ~Observer() = default;
+
+    // Called when we are done with the initial scan for printers.  We may
+    // still call OnPrintersFound if the set of available printers
+    // changes, but the user can conclude that if a printer is currently
+    // available and not in the list, we're not still looking for it.
+    virtual void OnDiscoveryInitialScanDone() = 0;
+
+    // Called with a collection of printers as they are discovered.  On each
+    // call |printers| is the full set of known printers; it is not
+    // incremental; printers may be added or removed.
+    //
+    // Observers will get an OnPrintersFound callback after registration
+    // with the existing list of printers (which may be empty) and will get
+    // additional calls whenever the set of printers changes.
+    virtual void OnPrintersFound(const std::vector<Printer>& printers) = 0;
   };
 
   // Static factory
-  static std::unique_ptr<PrinterDiscoverer> Create();
-
-  virtual ~PrinterDiscoverer() {}
-
-  // Begin scanning for printers.  Found printers will be reported to the
-  // attached observer.
-  virtual bool StartDiscovery() = 0;
+  static std::unique_ptr<PrinterDiscoverer> CreateForProfile(Profile* profile);
 
-  // Stop scanning for printers.  Returns false if scanning could not be stopped
-  // and new results will continue to be be sent to observers.  Returns true if
-  // scanning was stopped successfully.  No calls to the attached observer will
-  // be made after this returns if it returned true.
-  virtual bool StopDiscovery() = 0;
+  virtual ~PrinterDiscoverer() = default;
 
   // Add an observer that will be notified of discovered printers.  Ownership of
   // |observer| is not taken by the discoverer.  It is an error to add an
-  // oberserver more than once.
+  // observer more than once.  Calls to |observer| methods will take place on
+  // the thread PrinterDiscoverer was instantiated on.
   virtual void AddObserver(PrinterDiscoverer::Observer* observer) = 0;
 
   // Remove an observer of printer discovery.