This will enable more Vulkan-friendly idioms like clearing the
vulkan pipeline caches correctly on GL state changes immediately
because we have access to the ContextVk.

BUG=angleproject:1898

Change-Id: I16c848d8abdde8e26a38d384e565cec8548a66d0
Reviewed-on: https://chromium-review.googlesource.com/459079Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>