DM: add a pixel check to serialize

DM: add a pixel check to serialize

If we're running through serialize and then drawing into some raster target (e.g. serialize-8888 like we run on the bots), make sure the serialized version is identical to what we'd get if we just drew into the rest of the pipeline (i.e. 8888).

Start by blacklisting all current failures.  This at least prevents further regression.

BUG=skia:4095
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1561993003

Review URL: https://codereview.chromium.org/1561993003