Auto merge of #6572 - nnethercote:js-reporting, r=jdm

Wire up the JS engine's memory reporting.

SpiderMonkey provides an extremely fine-grained breakdown of memory
usage, but for Servo we aggregate the measurements into a small number
of coarse buckets, which seems appropriate for the current level of
detail provided by Servo's memory profiler. Sample output:
```
|      17.41 MiB -- url(file:///home/njn/moz/servo/../servo-static-suite/wikipedia/Guardians%20of%20the%20Galaxy%20(film)%20-%20Wikipedia,%20the%20free%20encyclopedia.html)
|          7.32 MiB -- js
|             3.07 MiB -- malloc-heap
|             3.00 MiB -- gc-heap
|                2.48 MiB -- used
|                0.34 MiB -- decommitted
|                0.09 MiB -- unused
|                0.09 MiB -- admin
|             1.25 MiB -- non-heap
```
Most of the changes are plumbing to get the script task communicating
with the memory profiler task.

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