This patch casts gl_InstanceID to uint before doing division by the
number of views to circumvent the HLSL compiler's warning on performance
degradation.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: I326530ee112f34f82becdec5239edd5054c4104f
Reviewed-on: https://chromium-review.googlesource.com/655298
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>