Vulkan: Refactoring in RenderPassCommandBufferHelper. This is a follow-up refactoring after CL: cae00d9bba91bd6857d71c07b853dba8485e2ab8 Vulkan: Cleanup shared ring buffer cmd alloc feature. "mCurrentSubpass" naming is a bit confusing. When custom Vulkan secondary command buffers are used, "mCurrentSubpass" is always "0". This is because custom command buffers allows storing multiple subpasses. This CL renames this member into "mCurrentSubpassCommandBufferIndex", to clearly state that it is refer to command buffer index of a current subpass. Custom command buffers will always use single buffer ("0"), while Vulkan secondary command buffers will have a buffer per subpass. Bug: b/256666069 Change-Id: I6777e3ca4ee57513e1c36f05f8d223eb086b095a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4227072 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com>