add isolate (init-with-previous) savelayer flag

BUG=skia:4884

Change-Id: If7fabf5cc2c87b870f48dfb87e27a2524fec5ae5
Reviewed-on: https://skia-review.googlesource.com/9045
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed