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:
```
|   10.99 MiB -- pages
|       7.75 MiB -- url(http://html5demos.com/worker)
|          4.63 MiB -- js
|             2.00 MiB -- gc-heap
|                0.94 MiB -- decommitted
|                0.92 MiB -- used
|                0.09 MiB -- unused
|                0.05 MiB -- admin
|             1.44 MiB -- malloc-heap
|             1.19 MiB -- non-heap
|          [...]
|       3.24 MiB -- url(http://html5demos.com/js/worker-cruncher.js)
|          3.24 MiB -- js
|             1.17 MiB -- malloc-heap
|             1.06 MiB -- non-heap
|             1.00 MiB -- gc-heap
|                0.69 MiB -- used
|                0.19 MiB -- decommitted
|                0.09 MiB -- unused
|                0.03 MiB -- admin
```
Most of the changes are plumbing to get the script and worker tasks
communicating with the memory profiler task.

components/compositing/pipeline.rs

@@ -318,6 +318,7 @@ impl PipelineContent {
                                   self.resource_task,
                                   self.storage_task.clone(),
                                   self.image_cache_task.clone(),
+                                  self.mem_profiler_chan.clone(),
                                   self.devtools_chan,
                                   self.window_size,
                                   self.load_data.clone());