Merge GrGLSLShaderVar and GrShaderVar

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5087

Change-Id: Ib8943a1da1ea495554feaf5b0992b94fbb9539ab
Reviewed-on: https://skia-review.googlesource.com/5087
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
55 files changed