This simplifies the growing strategy used in {ZoneBuffer} and also tunes
the initial sizes used for various instances of these buffers. Note that
such a {ZoneBuffer} is used for entire modules and individual function
bodies.

R=clemensh@chromium.org

Change-Id: I99a0898589984e1830c681845fabb0ed5f8317ab
Reviewed-on: https://chromium-review.googlesource.com/508711
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45419}