Dedup SkPaths by gen-ID when going to .skp

We're all set up to serialize and deserialze paths by an integer ID,
but we're just not deduping any paths (every path gets a new ID).

This turns on deduping.  This should mean the number of paths in a
deserialized SkPicture are the same as when it was recorded.

No diffs:
https://gold.skia.org/search2?issue=1409373011&unt=true&query=source_type%3Dgm&master=false

BUG=skia:4527

Review URL: https://codereview.chromium.org/1409373011
3 files changed