Fix the types of some of the ProfilerOverheadStats object fields

We have a type for Overhead statistics, and all its values are Microseconds right now: https://github.com/firefox-devtools/profiler/blob/dcb8c42af81c7ee6f14c1fa5be9881dc5d03c115/src/types/profile.js#L459-L482

But some of it’s values are not actually Microseconds. For example samplingCount is just a number and overheadPercentage is a percentage. So we should fix these types to reflect the actual values. For percentage, since we don’t have a Percentage type, we need to create that type under the InnerWindowID and will look pretty similar since it’s also going to use the number esentially: https://github.com/firefox-devtools/profiler/blob/dcb8c42af81c7ee6f14c1fa5be9881dc5d03c115/src/types/profile.js#L31

After making these changes, you should make sure flow and linter is happy by running yarn flow and yarn lint-js.

[canova]

Hi @negative0! Are you an Outreachy applicant?

If yes, great, we can assign it to you. But keep in mind that we have lots of applicants already and it might be better for you if you look for a project with less applicants at this point.

If not, sorry, but we are currently in the middle of the Outreachy contribution period and we don’t exactly have the bandwidth for contributors. But we would be more than happy if you want to contribute to the project once we are done with it. It will be done at the end of this month!

[NisaSource]

@canova Cool, I will start working on #2810 now. Thanks