This adds entrypoints for two new extensions that will be useful for
importing external Vulkan objects into ANGLE.

Bug: angleproject:3289

Change-Id: I206dc76eda5c6c8d836db7c6413c5544326aa722
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1552024
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>