Similar to how we cache the base common draw states. This will improve
DrawElements performance. Several state checks are optimized into a
single 'if' check of a cached value.

Also includes a regression test for mapping the element array buffer.

Bug: angleproject:2966
Change-Id: Ia6e524a58ad6b7df2e455d67733e15d324b1b893
Reviewed-on: https://chromium-review.googlesource.com/c/1357150Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>