Distribute gradient factory functions to the correct .cpp file(s)

Bug: skia:13438
Change-Id: I6ed1e2bfcfa79b66a191a8c403e00fa5732af8a3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/570826
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
5 files changed