The getLayers method was redundant, and the getBaseLevelImage method
we can replace with a shared TextureD3D method rather than a
specialized method for each type.

BUG=angle:729

Change-Id: I80c94a765dbc369fb0bb60ec8273e9e3ba6ffeed
Reviewed-on: https://chromium-review.googlesource.com/219862Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>