GPU conversion means we should be using a default of 1 as for
int types instead of the MAXINT value for normalized types.

Fixes the WebGL test for vertex attribs with offsets.

BUG=angle:550

Change-Id: I1aa1b1bae2282e8eda96366adeec492ffe1d5728
Reviewed-on: https://chromium-review.googlesource.com/188433Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>