If the vulkan driver has support for VK_KHR_external_memory_fd or
VK_KHR_external_semaphore_fd, add the GL versions of these to the vulkan
renderer's extensions.

Bug: angleproject:3289

Change-Id: I7f04b5cf883f93f6ccd579c2b75d6831b854bfd0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1552027
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>