udev_watcher: improving Observer class definition

OnDeviceAdded() and OnDeviceRemoved() abstract.
destructor being protected.

Change-Id: Ibc348d6565b6d2627ae53a34f05a63c08f68a8e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546308
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#646458}

device/udev_linux/udev_watcher.h

@@ -22,9 +22,11 @@ class UdevWatcher {
  public:
   class Observer {
    public:
+    virtual void OnDeviceAdded(ScopedUdevDevicePtr device) = 0;
+    virtual void OnDeviceRemoved(ScopedUdevDevicePtr device) = 0;
+
+   protected:
     virtual ~Observer();
-    virtual void OnDeviceAdded(ScopedUdevDevicePtr device);
-    virtual void OnDeviceRemoved(ScopedUdevDevicePtr device);
   };
 
   // subsystem and devtype parameter for