Set a name for WorkQueue tasks (fixes #1830)
auto merge of #1948 : mbrubeck/servo/task-name, r=jdm

This is my first Servo patch.  Please don't hesitate to offer basic Rust tips.

Note: This assigns the same name to all of the worker tasks for a given WorkQueue.  Would it be useful to generate a unique name for each tasks (LayoutWorker1, LayoutWorker2...)?