RenderThread: Make ~IFrameCallback() virtual

IFrameCallback has a pure virtual method, but its destructor was
non-virtual.  While CanvasContext, which inherits from
IFrameCallback, declares its own destructor virtual, this is not
sufficient when accessing via an IFrameCallback pointer.

We fix this by making the base destructor virtual.

Test: Treehugger
Change-Id: I6424f90c16caea417b5f80b60d76a7ca7c7ef97c

libs/hwui/renderthread/RenderThread.h

@@ -59,7 +59,7 @@ public:
     virtual void doFrame() = 0;
 
 protected:
-    ~IFrameCallback() {}
+    virtual ~IFrameCallback() {}
 };
 
 struct VsyncSource {