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/http_fetcher.h b/http_fetcher.h
index b8c6ad6..d291d6d 100644
--- a/http_fetcher.h
+++ b/http_fetcher.h
@@ -169,6 +169,8 @@
 // Interface for delegates
 class HttpFetcherDelegate {
  public:
+  virtual ~HttpFetcherDelegate() = default;
+
   // Called every time bytes are received.
   virtual void ReceivedBytes(HttpFetcher* fetcher,
                              const char* bytes,