Vulkan: Remove API access for mTotalBufferToImageCopySizen Don't expose mTotalBufferToImageCopySize via API if it is unnecessary, for better encapsulation. Bug: angleproject:6354 Change-Id: If69a5a78cc40d54feee6f3474aef6292d60b89c5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3609016 Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Commit-Queue: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>