Vulkan: Store Pipeline/DS Layouts in ProgramVk. We can keep a shared reference to the Pipeline and Descriptor Set layouts in the Program. This ensures they are not in use when they are deleted. Note that they are allowed to be deleted as long as no command buffers are currently recording with them. If the Program is deleted then there should be no further commands using these layouts. Bug: angleproject:2462 Change-Id: I75161b3ce1ee8eae33dd6becee79b4262b844cdd Reviewed-on: https://chromium-review.googlesource.com/1089807 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>