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/1456478Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>