The stack chart's view for allocations data isn't finished

[julienw]

In a recent PR (https://github.com/firefox-devtools/profiler/pull/3308) I made the stack chart work for allocations data. However I noticed some issues:

• the tooltip still shows timing data -- I'm not even sure it is correct, it may be displaying allocation data with timing units...
• it's also possible that the stacks are incorrect, because I'm not sure the right thread/sample table objects are being used.

So in this issue we want to double check this.

Here is an example profile

┆Issue is synchronized with this Jira Task