Make BBH::search() const.

I'd like to have SkRecordDraw() work with a const SkBBoxHierarchy*, but
can't quite today.  The only interesting change here is no longer flushing
if needed in RTree; instead we assert we've been flushed already.

BUG=skia:
R=robertphillips@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/452233002
7 files changed