This will be necessary to support more than one PipelineLayout per
instance of ANGLE. Sampler array handling requires different layouts
for different sampler uses.

Bug: angleproject:2462
Change-Id: I1d8b4919eed1a589002ad1898b05186f420061c7
Reviewed-on: https://chromium-review.googlesource.com/1089806
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>