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>

action_processor.h

diff --git a/action_processor.h b/action_processor.h
index b3cc67b..121e0e1 100644
--- a/action_processor.h
+++ b/action_processor.h
@@ -80,6 +80,8 @@
 // ActionProcessor to register itself.
 class ActionProcessorDelegate {
  public:
+  virtual ~ActionProcessorDelegate() = default;
+
   // Called when all processing in an ActionProcessor has completed. A pointer
   // to the ActionProcessor is passed. |code| is set to the exit code of the
   // last completed action.