NativeImageSource --> NativeImageInterface is simple valid job.
NativeImageInterface --> NativeImageSource is not simple and expensive job.
WebEngine / WebPlugin defined at dali-adaptor/devel-api and
NativeImageSource defined at dali-adaptor/public-api.
So there is no reason to convert NativeImageSource to NativeImageInterface.
Change-Id: I55e9c6824188b1df8b53ea50b4f638f9309cfd84
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>