Ensure gl State structs are zero filled. In some cases we would hash or memcmp against structs with bools or other non-filled data. This could have implementation differences, and may have been causing cache errors on Clang. BUG=chromium:721648 BUG=angleproject:2044 Change-Id: I981a1e6e8d50a33f7fade568497b72b919accfce Reviewed-on: https://chromium-review.googlesource.com/516383 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>