explicitly bump legacy genID on gpu surface

Previous attempt to "test" getTextureHandle was giving false-positive because we had an active image-snapshot, which was incidentally also triggering a new genID.

I think this CL will also enable this change: https://codereview.chromium.org/1222243002/

BUG=skia:
TBR=bsalomon@google.com
NOTREECHECKS=True

Review URL: https://codereview.chromium.org/1208993008