Convert XYZ values from PNGs to D50

Previously, we were storing whatever values we got out of the file
and pretending they were D50.  Brian has demonstrated that this
doesn't really work.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2180483003

Review-Url: https://codereview.chromium.org/2180483003
1 file changed