Vulkan: Add stubs for expose VK_KHR_swapchain_mutable_format Bug: angleproject:386688871 Change-Id: I77a45e342ee46f65c4f2947071c713f914a937a2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6196939 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>