Bug 1181452 - Add memory reporting infrastructure for Servo to SpiderMonkey. r=till.

Basically, this change is all about aggregating SpiderMonkey's fine-grained
measurements into the new set of coarse-grained measurements, called
ServoSizes (which is similar to the existing TabSizes). The change utilizes and
extends the existing macro machinery to do this in a way that has some chance
to maintaining correctness over the long-term despite the fact that this code
is so fiddly.

--HG--
extra : rebase_source : 1dc3d92830902d9e24496dcdc5f1ee8a6fe39fb4

Author: nnethercote