Refactor `cpu_profile_model` to prevent side effects

[elliette]

The functions called in generateFromCpuSamples must be called in that particular order. This is because of function side effects:

1. Generating the stack frames both creates the timeline tree and assigns frame IDs to every node in the tree
2. Creating the sample events expects that the tree has already been created, and that the frame IDs have been assigned to every node

Ideally we should refactor the code so that we aren't dependent on hidden side effects.