Clean up Buffer11. This cleans up some messy stuff from the emulated index buffers, which were caching variables that didn't need to be cached. Also add in missing error checks. This touches a lot of code. BUG=angleproject:1327 BUG=angleproject:1310 Change-Id: Icd722d57d9449388fbabc62c7ea37f0526a568ff Reviewed-on: https://chromium-review.googlesource.com/334731 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>