fix incorrect bounds on remove child from root

Before this patch it was possible that the root node would have incorrect bounds after removal of a direct descendant node. Fixes that and adds a unit test to detect it in the future.

BUG=373255
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=270605

[luken, 2014-05-14 20:17:20]

Came up when I was fuzzing RTree inserts and deletes to try to reproduce the
crash on the linked bug.

[piman, 2014-05-14 21:35:12]

lgtm

[sky, 2014-05-14 22:34:09]

Rubber stamp LGTM

[luken, 2014-05-14 22:34:39]

The CQ bit was checked by luken@chromium.org

[commit-bot: I haz the power, 2014-05-14 22:36:03]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/luken@chromium.org/278403004/2

[commit-bot: I haz the power, 2014-05-15 00:06:14]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-15 06:56:59]

Change committed as 270605