Added gradient shader factories that take SkColor4f + SkColorSpace.
Modified Descriptor to only store SkColor4f + SkColorSpace.
Existing factories make use of helper code to convert SkColor and
forward to the new factories.
Bumped SKP version to handle new gradient serialization format.
I was toying with using half-float when serializing SkColor4f,
despite my aggressive packing of flags, this format is significantly
bigger.
Also added GM to use 4f factories. This GM should (and does)
look identical to the existing gradients GM.
BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2370063002
Review-Url: https://codereview.chromium.org/2370063002
e25d71ccbc