GL_OES_depth_texture_cube_map depends on depthTextureOES,
but adds the same new texture format/type requirements as
GL_OES_packed_depth_stencil, so instead of adding a
specific check for GL_OES_depth_texture_cube_map, the
Vulkan backend checks for a combination of
GL_OES_depth_texture and GL_OES_depth_texture_cube_map
to enable GL_OES_depth_texture_cube_map.

ANGLE has no specific checks for whether a format is
"cubemappable", but Vulkan has no restrictions of the
format of cubemaps, so if the proper formats are
supported, creating a cube of any of these formats
should be implicitly supported.

Bug: angleproject:4293
Change-Id: I968dbe8869ba0f50de18dd41f1195e847c06b520
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2026027
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>