Add a virtual destructor to PermissionPromptDelegate

This is needed so that subclass destructors will be called when
destroying a PermissionPromptDelegate pointer. This currently has no
impact on behavior since there are no subclass destructors.

BUG=596786

Review-Url: https://codereview.chromium.org/2836093006
Cr-Commit-Position: refs/heads/master@{#467617}

chrome/browser/media/webrtc/media_stream_devices_controller.h

@@ -91,6 +91,7 @@ class MediaStreamDevicesController {
         bool user_gesture,
         content::WebContents* web_contents,
         std::unique_ptr<MediaStreamDevicesController::Request> request) = 0;
+    virtual ~PermissionPromptDelegate() {}
   };
 
   static void RequestPermissions(