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.

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