Don't store legacy colors in gradient shaders

We only pass linear/4f colors to the ctor, and then derive the legacy
colors from them.

Might as well just derive when needed.

Change-Id: I82b3d159da91f6faa4a3e7d681763c0ec1cdab07
Reviewed-on: https://skia-review.googlesource.com/65680
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>
6 files changed