Fix race condition in BumpPointerSpace::Walk.

There was a race where we would incorrectly tread part of the main
block as other blocks. This was caused by a thread allocating into
the bump pointer space when another thread was walking it. The new
solution is to ignore objects allocated since we started the walk.

Bug: 12966354
Change-Id: I49abd4de97609e9c9a3fae40b9c159abfdbbd07c