Use virtual destructors

It turns out to be just a comment, as all except AudioMixer are RefBase.

There are only a few performance-sensitive cases where it's worth thinking
about whether you need a virtual destructor, and the headache usually
outweighs the benefit.

Change-Id: I716292f9556ec17c29ce8c76ac8ae602cb496533
diff --git a/services/audioflinger/AudioMixer.h b/services/audioflinger/AudioMixer.h
index 84f6330..40a508e 100644
--- a/services/audioflinger/AudioMixer.h
+++ b/services/audioflinger/AudioMixer.h
@@ -33,7 +33,7 @@
 public:
                             AudioMixer(size_t frameCount, uint32_t sampleRate);
 
-                            ~AudioMixer();
+    /*virtual*/             ~AudioMixer();  // non-virtual saves a v-table, restore if sub-classed
 
     static const uint32_t MAX_NUM_TRACKS = 32;
     static const uint32_t MAX_NUM_CHANNELS = 2;