Small fix for PresentationScreenAvailabilityListener.

Also fixed PresentationServiceAvailabilityListener interface.
- fixed dtor naming and moved to public
- GetPresentationUrl() is now a const function

BUG=412331

Review URL: https://codereview.chromium.org/887293002

Cr-Commit-Position: refs/heads/master@{#314095}

content/public/browser/presentation_screen_availability_listener.h

@@ -14,15 +14,14 @@ namespace content {
 // See also PresentationServiceDelegate.
 class PresentationScreenAvailabilityListener {
  public:
+  virtual ~PresentationScreenAvailabilityListener() {}
+
   // Returns the Presentation URL associated with this listener.
-  virtual std::string GetPresentationUrl() = 0;
+  virtual std::string GetPresentationUrl() const = 0;
 
   // Called when screen availability for the associated Presentation URL has
   // changed to |available|.
   virtual void OnScreenAvailabilityChanged(bool available) = 0;
-
- protected:
-  virtual ~ScreenAvailabilityListener() {}
 };
 
 }  // namespace content