Revert "remove 'in uniform' support from GrSkSLFP, make rules more clear"

This reverts commit ac18a5ca60d47c26ada431b49373f7b9371686a5.

Reason for revert: breaking Chrome roll

Original change's description:
> remove 'in uniform' support from GrSkSLFP, make rules more clear
> 
> Bug: skia:
> Change-Id: Iaa4d33c1bfb295d87343411ba6aacc8fae68ef9c
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244300
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

TBR=brianosman@google.com,ethannicholas@google.com

Change-Id: I6e53f5197c751d961abfa21861b940d4168de213
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244508
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
11 files changed