Vulkan: Add support for loading layers from DisplayVk Fuchsia needs a layer to support VK_KHR_swapchain. Add the ability for DisplayVk subclasses to request a layer in addition to an extension. BUG=angleproject:2475 TEST=angle_end2end_tests on Fuchsia Change-Id: If86c773d4bd4ebecaee4b1631d3d4975105fe849 Reviewed-on: https://chromium-review.googlesource.com/c/1456478 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>