Repairs crash in RTreeBase::Node::LeastAreaEnlargement

Repairs crash in RTreeBase::Node::LeastAreaEnlargement

This CL was originally uploaded at
https://codereview.chromium.org/269513002 but was reverted due to a high
number of crash reports on Windows. The issue was that RTree::Insert()
would remove duplicate nodes before re-inserting them, but didn't handle
the corner case where the root node would end up as a non-leaf node with
only one child. RTree::Remove() did the right thing in this case, which
is to remove the root and replace it with its solitary child, but
RTree::Insert() did not. This lead to a situation with invalid trees
being created that would ultimately lead to a crash on a subsequent
call to RTree::Insert(). A short unit test that reproduced the crash
is included.

BUG=384736
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=278227

[luken, 2014-06-18 16:47:33]

re-uploaded patch with original reverted patch first, so you can see the diff.

[sky, 2014-06-18 17:24:09]

So much easier to see what actually changed. LGTM

[luken, 2014-06-18 17:25:24]

The CQ bit was checked by luken@chromium.org

[commit-bot: I haz the power, 2014-06-18 17:26:18]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/luken@chromium.org/342723002/20001

[commit-bot: I haz the power, 2014-06-19 01:30:05]

Change committed as 278227