Add a toggle option to view unmangled asm.js & wasm C++ callstacks in profiling view?

Profiling [Zen Garden](http://mzl.la/webassemblydemo) demo with gecko profiler shows C++ function names as mangled:


Currently the built-in Firefox devtools performance profiler displays these C++ callstacks as demangled:


which is really nice and a bit more readable. It would be cool to have the same feature in perf.html, however that feature would be best to live behind a checkbox (default enabled), because it is often useful also to be able to select and copy-paste mangled function names from the profiler to a text editor to search for the hot function in compiled code, so unconditionally always showing unmangled C++ function names would lose that ability.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-171)