Vulkan: Add overlay widget for cache key size. Bug: angleproject:6776 Change-Id: I35ab18bc5919129b2decf58d541499f771140e47 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3472754 Reviewed-by: Yuxin Hu <yuxinhu@google.com> Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>