update_engine: Enforce virtual destructors on virtual classes.

This patch enables -Wnon-virtual-dtor enforcing that virtual classes
such as interfaces have a virtual destructor. This is required by
the Google Coding Style and avoids some problems where the derived
class' destructor is not called.

BUG=None
TEST=FEATURES=test emerge-link update_engine

Change-Id: Id907e3c14923fcccc20b83bd064fa9c9c51fffb3
Reviewed-on: https://chromium-review.googlesource.com/228927
Commit-Queue: Alex Deymo <deymo@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/download_action.h b/download_action.h
index 8e3809a..6f16e20 100644
--- a/download_action.h
+++ b/download_action.h
@@ -28,6 +28,8 @@
 
 class DownloadActionDelegate {
  public:
+  virtual ~DownloadActionDelegate() = default;
+
   // Called right before starting the download with |active| set to
   // true. Called after completing the download with |active| set to
   // false.