Show commit priority levels in Profiler UI (if available)

[bvaughn]

Note that this PR is blocked by React PR 15664 and unit tests are expected to fail unless using a version of React built from from the devtools-commit-priority-level branch.

What was the priority of this commit?

Some commits are high priority. Others are pre-rendering offscreen work. Up until now though, the Profiler has treated them all the same.

Here's an example app:

function PriorityLevels() {
  const [count, setCount] = useState(0);

  const startSequence = useCallback(() => {
    setCount(1);
    next(() => setCount(2));
  }, []);

  return (
    <Fragment>
      <h1>Priority Levels</h1>
      <button onClick={startSequence}>start sequence</button>
      {count >= 1 && <Text>One</Text>}
      {count >= 2 && <Text>Two</Text>}
      {count >= 2 && (
        <div hidden>
          <Text>Three</Text>
        </div>
      )}
    </Fragment>
  );
}

const Text = ({ children }) => children;

And here's what the UI could look like: