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

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:

Committed: https://skia.googlesource.com/skia/+/208d1704c2cc1161ea337a05155f2b9a04edd351

[mtklein, 2014-10-08 18:23:59]

mtklein@google.com changed reviewers:
+ mtklein@google.com, robertphillips@google.com

[mtklein, 2014-10-08 18:25:11]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-10-08 18:25:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/639933003/1

[commit-bot: I haz the power, 2014-10-08 18:25:26]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from valid reviewer(s) till 2014-10-09 00:25 UTC

[robertphillips, 2014-10-08 18:51:31]

lgtm

[commit-bot: I haz the power, 2014-10-08 21:17:31]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-10-08 21:17:32]

Failed to apply the patch.

[mtklein, 2014-10-09 13:37:54]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-10-09 13:38:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/639933003/20001

[commit-bot: I haz the power, 2014-10-09 13:49:51]

Committed patchset #2 (id:20001) as 208d1704c2cc1161ea337a05155f2b9a04edd351