Vulkan: Expose EXT_debug_marker and stub out the implementation. This extension is used by Chrome. BUG=angleproject:2853 Change-Id: Ie8d4ba07857c581a4ec3f90faabdf747b6b52445 Reviewed-on: https://chromium-review.googlesource.com/1246263 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>