refactored SkSLVarDeclaration out of existence

Bug: skia:
Change-Id: I3dbc08e6d759f6828a472246d4797babb6cc132e
Reviewed-on: https://skia-review.googlesource.com/66147
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
16 files changed