We were incorrectly setting the signed parameter to true
in the vertex format table. This fixes a dEQP test in
vertex_arrays.output_types.

BUG=angle:586

Change-Id: I84dcdee89a95369cc8e41d451f96dbb8daa37b6d
Reviewed-on: https://chromium-review.googlesource.com/190056Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>