The robustness spec states that all GL calls should generate context
lost errors when the context is lost. This behaviour was lost due to
some context lookup optimizations but can be re-added at no cost by
updating the autogenerated entry points.

Updated the ContextLostTest to cover the repeated error generation
behaviour.

Bug: chromium:1137241
Change-Id: I0b8d1cf4d2a37f13c466c8dd0e66edfe852f7e59
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2488180Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>