This map is often quite small and holds small items (ints) so wastes
quite a bit of overhead in the backing tree representation.

This CL changes the std::map to a sorted vector of pairs. This reduces
the size significantly (2.13 MiB -> 598 KiB on the node server example).

Bug: v8:7719
Change-Id: Ic829693f007732ae145fae02850a1ed913cd941e
Reviewed-on: https://chromium-review.googlesource.com/1064233
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53278}