Only enable VK_KHR_maintenance1 when available. This prevents an error in case ANGLE is run on an older Vulkan driver that does not support the VK_KHR_maintenance1 extension. Bug: angleproject:3035 Change-Id: I7e2b74bcb88bffdfc2a525a1d332eb3ef7d23d7c Reviewed-on: https://chromium-review.googlesource.com/c/1387347 Reviewed-by: Ian Elliott <ianelliott@google.com> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com> Commit-Queue: Ian Elliott <ianelliott@google.com>