SkPdf: smaller color serialization

SkPDFUtils now has a special function (SkPDFUtils::AppendColorComponent)
just for writing out (color/255) as a decimal with three digits of
precision.

SkPDFUnion now has a type to represent a color component.  It holds a
utint_8, but calls into AppendColorComponent to serialize.

Added a unit test that tests all possible input values.
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2151863003

Review-Url: https://codereview.chromium.org/2151863003
8 files changed