Add SkBBoxHierarchy::reserve() as an optional size hint.

I want to play around with how SkTileGrid stores its tiles.  Having a
cap on the number of insert() calls can be pretty handy.

While I'm at it, I gave flush() a default empty impl.  Like reserve(),
it's really an optional hook for subclasses.

BUG=skia:

Review URL: https://codereview.chromium.org/639933003
5 files changed