Vulkan: Change ContextVk to Context for BufferPool APIs These APIs only needs information from Context, not ContextVk. This CL changes to Context for better encapsulation. Bug: b/223428306 Change-Id: I4f50aaa4065eff62ca32e9049f5a891d8814e511 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3578587 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Lingfeng Yang <lfy@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>