Doesn't provide `PerformanceEntry.toJSON` when unsupported

[ScubaDaniel]

Using IE11, the performance.getEntries() is supported, but PerformanceEntry.toJSON() is not. toJSON is only added when one of the polyfill methods is applied, so the polyfill doesn't help support IE11.

[ScubaDaniel]

Additionally, why does getEntries only get added if performance.mark or performance.measure is unsupported?

Opera doesn't support getEntries, but supports the other two. Wouldn't that essentially make this polyfill not apply? https://developer.mozilla.org/en-US/docs/Web/API/Performance