add SkGraphics::PurageAllCaches

BUG=skia:4755
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1722603002

TBR=

Review URL: https://codereview.chromium.org/1722603002
2 files changed