Vulkan: Initialize exectuable with invalid uniform serial. Instead of reserving valid serials from the range of device serials, we can instead use an invalid serial. We can remove the extra check for valid this adds if it becomes a hotspot later. Bug: angleproject:6776 Change-Id: I81a36be503d6464ddc5dbb0e9df907f09dda9182 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3426883 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>