Remove 'friend SkRefCnt' from SkData.

Remove 'friend SkRefCnt' from SkData.

[bungeman-skia, 2013-04-15 22:34:25]

So the issue we were having (which motivated the 'friend') is that if a non-POD
class does not provide a default destructor, one is provided. GCC will do so,
but only at the point where the vtable is output, since this class is never
needed the destructor body is never output, so there is no complaint. VC++,
however, gladly provides the destructor implementation as soon as it sees the
type. If the destructor is ever defined then you get an error (since the base
class destructor is private).

This change corrects two things. First is that it declares (but does not define)
a destructor for BlockRef, which prevents a default one from being provided and
therefore removes the VC++ error. Also, it now also blocks access to ever
calling any destructor through '->' which is what we want as well. This means we
can finally completely clean up the SkAutoDataUnref to simply be
SkAutoTUnref<SkData>.

[reed1, 2013-04-16 11:49:23]

lgtm

[bungeman-skia, 2013-04-16 15:49:50]

Committed revision 8697.