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>