add explicit bench for raw bitmapscaler

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1586803003

patch from issue 1586803003 at patchset 20001 (http://crrev.com/1586803003#ps20001)

TBR=

Review URL: https://codereview.chromium.org/1588113002
2 files changed