Wasm frames are shown as "?"

Taking a gecko profile of the [Zen Garden](http://mzl.la/webassemblydemo) page shows the following


The frames marked with `?` in the profile are WebAssembly frames. The `entry trampoline` line is a JS to Wasm transition (there is no asm.js in the page), and the `fast FFI trampoline` line is a Wasm to JS transition.

Though I doubt there is much preference or need to differentiate between what is asm.js and what is Wasm, so perhaps just labelling all of that code with `asm.js/wasm`, or just `wasm` might make most sense, since asm.js will slowly go away as WebAssembly begins to roll in.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-176)