clean up SkShader friends

Many SkShader subclasses are only friends to use makeColorSpace().

Our usual solution to this is to add a method on SkColorSpaceXformer
that calls makeColorSpace() for us, so that only SkColorSpaceXformer
needs to be a friend.

Just a refactor.  No image diffs.

Change-Id: Icf952b739edf45f2fb8c0c35e353ef2866f4c5cc
Reviewed-on: https://skia-review.googlesource.com/16370
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
7 files changed