Allow to compare two profiles for their memory usage

While the profiler allows to compare two profiles for code execution (stack frames), it's not possible to actually compare the memory usage. I have to do such a comparison for https://bugzilla.mozilla.org/show_bug.cgi?id=1669169#c16.

Here two profiles that I want to compare: https://share.firefox.dev/32SgQrR and https://share.firefox.dev/2H5gx5g

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-342)

No source files or tests are named. Start by examining the existing profile-comparison flow for stack frames and load the two linked profiles to determine how their memory usage should be compared; done means the profiler supports a useful memory comparison for those profiles and has coverage for the behavior.

Written by the indexing model from the issue text.