Record concat as Concat.

This seems to cause very slight diffs on GMs.
https://gold.skia.org/search2?issue=1462983002&unt=true&query=source_type%3Dgm&master=false

I'm not sure I understand why.
The diffs to at least pictureshader look like improvements.

BUG=skia:4584

Review URL: https://codereview.chromium.org/1462983002
4 files changed