- Instead of tracing during marking we can now trace in a separate phase in MC.
  (Heap is iterable.)
- Add more subtypes for fixed arrays, reducing the unknown bucket to around ~8%
  (local run).
- Refactor collection calls to have a single bottleneck.
- Provide JSON-based output format that can be "easily" processed in JS.

BUG=
R=ulan@chromium.org,hpayer@chromium.org

Review-Url: https://codereview.chromium.org/2129173002
Cr-Commit-Position: refs/heads/master@{#37718}