Enable instanced draws with the Vulkan backend.
So far it only works when Vulkan has VK_EXT_vertex_attribute_divisor.

BUG=angleproject:2672

Change-Id: Ib6655625776344305911a1a742c85f17638cee8f
Reviewed-on: https://chromium-review.googlesource.com/c/1469263Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>