We were incorrectly calling Texture::getBaseLevelDepth() which was
correctly reporting the number of 3D slices of the texture (1).

Also rename Texture2DArray::getDepth to getLayers for clarity.

Change-Id: I1968cbcb0a3528bb76ec3e4ea4148e5dc0f302c7
Reviewed-on: https://chromium-review.googlesource.com/181553Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>