platform2: Enforce virtual destructors on base classes

There have been a number of memory leak issues due to absence of
virtual destructors on base classes. Swept through the following targets:
buffet, libchromeos, webserver.

For every class defined, I did one of the following three things:

1. If the class is not meant to be derived from, marked it as 'final'.
2. If classes derived from a particular base class are not meant to be
   deleted through the base class, marked the base class's destructor
   as 'protected'.
3. Otherwise made the base class's destructor virtual.

BUG=None
TEST=`FEATURES=test emerge-link libchromeos webserver buffet`

Change-Id: I4d909399896d025c39980c9546b79b145614fc47
Reviewed-on: https://chromium-review.googlesource.com/273000
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/commands/schema_utils.h b/buffet/commands/schema_utils.h
index 625b854..59e3b39 100644
--- a/buffet/commands/schema_utils.h
+++ b/buffet/commands/schema_utils.h
@@ -43,7 +43,7 @@
 // is inherited or overridden, while |is_inherited| can be used to identify
 // if the attribute was inherited (true) or overridden (false).
 template<typename T>
-class InheritableAttribute {
+class InheritableAttribute final {
  public:
   InheritableAttribute() = default;
   explicit InheritableAttribute(T val)