1) Rename {size()} to {total_size()} because
   {size() != end() - start()}.
2) Avoid undefined behaviour when creating segments, i.e. construct
   them via a constructor.
3) Minor drive-by cleanups.

R=mstarzinger@chromium.org

Bug: v8:8834
Change-Id: I3de47b2a775cf277e2f01ba5482afbd400acd06c
Reviewed-on: https://chromium-review.googlesource.com/c/1493926
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59950}