Refactor GrLayerHoister::FindLayersToHoist to prep for adding clip to cache key

NOTRY=true

Committed: https://skia.googlesource.com/skia/+/27415b71bd529456165945e19b5b7efbebf6fb51

Review URL: https://codereview.chromium.org/640373002
Remove unused SaveLayerInfo::fValid field

Minor code cleanup

Review URL: https://codereview.chromium.org/643713002