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/connection_manager.h b/connection_manager.h
index c77adee..f9973a3 100644
--- a/connection_manager.h
+++ b/connection_manager.h
@@ -37,6 +37,7 @@
   // Constructs a new ConnectionManager object initialized with the
   // given system state.
   explicit ConnectionManager(SystemState* system_state);
+  virtual ~ConnectionManager() = default;
 
   // Populates |out_type| with the type of the network connection
   // that we are currently connected and |out_tethering| with the estimate of