Add SkRecord::defrag().

Called by SkRecordOptimize(), this moves all the NoOps to the end and
slices them off.

This implementation with std::remove_if() is linear and doesn't malloc.

No diffs: https://gold.skia.org/search2?issue=1461663003&unt=true&query=source_type%3Dgm&master=false

BUG=skia:

Review URL: https://codereview.chromium.org/1461663003
5 files changed