The Context needed for the error refactor and Vulkan implementation.

Bug: angleproject:2738
Change-Id: Id816b61ebf342c953a320ab29b6386e356dc64e8
Reviewed-on: https://chromium-review.googlesource.com/1150092Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>