Controlled experiment to assess performance of module storage

Accompanies Id2835eca4. Logs module load timing data from browsers that are in
the experiment or control group.

See <https://meta.wikimedia.org/wiki/Schema:ModuleStorage>.

Change-Id: Ib9f4a56153066741bdae352910433e214ebcb621
Merge "Controlled experiment to assess performance of module storage"