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`.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-316)