This allows the GL backend to import Vulkan resources.

BUG=angleproject:3656

Change-Id: Ie5e55ce3e1ba05e917619e3f192c13dcc36c3739
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1688507Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Clemen Deng <clemendeng@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>