gpu: fix off-by-one in command_info array

The command_info array was 1 too large, causing a check to accept an invalid
command. Renamed kStartPoint to be less confusing.

BUG=None
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel;tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2000273002
Cr-Commit-Position: refs/heads/master@{#395395}
5 files changed