add test for copy-on-write optimization

exercises a bug in the device-clip CL (hopefully to land soon)
BUG=skia:

Change-Id: I5a740280e1a3d6a4634338d04d9f1a1f9dc84dbb
Reviewed-on: https://skia-review.googlesource.com/9325
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
1 file changed