Refactored SkColorSpace and added in a Lab PCS GM

Refactored SkColorSpace and added in a Lab PCS GM

The refactoring breaks off A2B0 tag support into a separate
subclass of SkColorSpace_Base, while keeping the current
(besides CLUT) functionality in a XYZTRC subclass.

ICC profile loading is now aware of this and creates the A2B0
subclass when SkColorSpace::NewICC() is called on a profile
in need of the A2B0 functionality.

The LabPCSDemo GM loads a .icc profile containing a LAB PCS and
then runs a Lab->XYZ conversion on an image using it so we can
display it and test out the A2B0 SkColorSpace functionality,
sans a/b/m-curves, as well as the Lab->XYZ conversion code.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2389983002

Review-Url: https://codereview.chromium.org/2389983002