add virtual ~Effect()

Today we make an sk_sp<Effect> from an sk_sp<EffectImpl>.
But when the sk_sp<Effect> dies, it calls ~Effect(), not ~EffectImpl().
Making ~Effect() virtual fixes this.

This should make our Google3 tests sized-delete clean, unblocking those folks.

Review URL: https://codereview.chromium.org/1769093002

tests/RefCntTest.cpp

@@ -99,6 +99,7 @@ public:
     Effect() : fRefCnt(1) {
         gNewCounter += 1;
     }
+    virtual ~Effect() {}
 
     int fRefCnt;
 
@@ -135,6 +136,8 @@ public:
 };
 
 struct EffectImpl : public Effect {
+    ~EffectImpl() override {}
+
     static sk_sp<EffectImpl> Create() {
         return sk_sp<EffectImpl>(new EffectImpl);
     }