Convert gradient params test helper to a struct

Much less copy-pasted code, fewer implementation details leaking out,
and going to be easier to extend for 4f and color space testing.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2900

Change-Id: Icc468c606aa35fbe82c64bcc398e7e348e0faa20
Reviewed-on: https://skia-review.googlesource.com/2900
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed