Vulkan: Use push constants in PipelineLayoutCache. This removes the internal pipeline layout for the masked clear shaders. Instead use the PipelineLayoutCache. Bug: angleproject:2462 Change-Id: I8f8365b866098ece3e964fd12447dfdea55c20ba Reviewed-on: https://chromium-review.googlesource.com/1090758 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>