add SkPicture::MakePlaceholder()

Bug: skia:7536

Change-Id: I6ca7c680ef4fd69419254dc7f1af27343dbb8e89
Reviewed-on: https://skia-review.googlesource.com/99664
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed