Use a bigger block size in the bump-pointer allocators, since we
basically know up front that we'll need lots of memory.  The 1MB value
(versus the default of 4KB) was chosen somewhat arbitrarily, and
succeeds in pretty much removing bump-pointer related mallocs from the
profile.

Pre-reserve the a priori known number of edges in getTerminatorEdges()
to avoid vector resizing.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/760973002