Without this, developers have to override `onReportTimings` to listen for `FrameTiming`.
That can potentially break previous `onReportTimings` listeners if they forget to call
the old listener in their new callback.
This PR replaces the similar RP in the framework: https://github.com/flutter/flutter/pull/38574
Once this PR landed, we'll have to create another framework PR to use the stream to replace
`onReportTimings` usages.
Once that's done, we can then propose the breaking change of removing the deprecated
`onReportTimings`.