glGetVertexAttribIuiv previously returned an incorrect value
for GL_VERTEX_ATTRIB_ARRAY_DIVISOR, because the divisor was
treated as an integer even though it is an unsigned integer.

BUG=angleproject:2281

Change-Id: I4d1c9df15244db855c6a000fc8fb6cc786bfcb85
Reviewed-on: https://chromium-review.googlesource.com/c/1273300Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>