Don't compute path keys for volatile paths in GrShape.

Otherwise, we will compute cache keys for internally transformed paths that don't repeat (e.g. clip paths transformed into device space with a changing view matrix).

BUG=chromium:649562
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2369513002

Review-Url: https://codereview.chromium.org/2369513002
3 files changed