Relocate gradient negative fract() workaround to GLSLCodeGenerator

Change-Id: If5aba5b266f86f677b6e63b0f79792f1d3213336
Reviewed-on: https://skia-review.googlesource.com/32202
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed