This Cl updates the cube example to use unique names for the entry
points and fixes the duplicate name used for fragColor between the
vertex and fragment shaders.
Change-Id: Icb77897bac88f7641cab2702b37573bfd771a586
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/24781
Reviewed-by: David Neto <dneto@google.com>