Instead of casting to the specific TextureStorage9 type before calling
getSurfaceLevel, merge the prototypes into a pure method in
TextureStorage9.

BUG=angleproject:970

Change-Id: I07b24b6cc470fc8cb6ba9bd9a8f178c3b43be36a
Reviewed-on: https://chromium-review.googlesource.com/293270Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>