We were emitting the table index once in the element section instead of
once per element segment. This did not cause failures because we never
had more than one element segment.
This CL fixes this, and adds a test for more than one segment.

R=rossberg@chromium.org

Change-Id: I13923baf3dae383c72760286e24242d0ad55c4db
Reviewed-on: https://chromium-review.googlesource.com/527155Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45857}