use onPeekBitmap instead of asLegacyBitmap

Since we know we are raster-backed at this point, it is fine, plus it avoids the copy-mutable logic in asLegacyBitmap, which is not needed here (and is slower if our bitmap was built with kNever_SkCopyPixelsMode).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2223653002

Review-Url: https://codereview.chromium.org/2223653002
1 file changed