[heap] reorder root list items.

- Moves the most often used 32 items to the top. This shaves off 10kB
  from the startup snapshot size.
- Sorts the rest by usage and type.

R=mlippautz@chromium.org

Review-Url: https://codereview.chromium.org/2088023003
Cr-Commit-Position: refs/heads/master@{#37176}