Vulkan: s/ContextVk/Context in pipeline creation With VK_EXT_graphics_pipeline_library, pipeline creation may happen on a thread. This change prepares the interface such that only a vk::Context is needed, instead of ContextVk. Bug: angleproject:7369 Change-Id: Ib7e9e7e140e27a4af71bffee069c88e3d7f47dfa Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3956935 Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>