Fix SkOneShotDiscardablePixelRef to return discardable segment.

The SkOneShotDiscardablePixelRef was not overriding
diagnostic_only_getDiscardable method of SkPixelRef. This caused the
memory to be acocunted under malloc instead of discardable and caused
wrong accounting.

BUG=528295

Review URL: https://codereview.chromium.org/1304083006
1 file changed