Update GrClampFragmentProcessor to use a child FP.

We are updating FPs to receive their input via a child FP where
possible, instead of relying on the input color.

Change-Id: Ic30224a45c6e4dba46c1cbaa27735013e8b3935f
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293937
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
6 files changed