Vulkan: Cache object pointers in ContextVk. Small performance optimization. Bug: angleproject:2786 Change-Id: Ib01c1762d438ffb72ec25bf05135a7d3271d6aee Reviewed-on: https://chromium-review.googlesource.com/1188954 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>