1) Format switch/case with git cl format
2) Change NULL -> nullptr
3) Change int to GLint where appropriate
4) Use more appropriate variants of asserts

BUG=angleproject:1261
TEST=angle_perftests

Change-Id: I97d6260dbd72ef3897fff145682b7043bf323b76
Reviewed-on: https://chromium-review.googlesource.com/319380
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>