Fiber logs timing for HostRoot if logTopLevelRenders enabled

[bvaughn]

Adds calls to console.time and console.timeEnd before beginning work on the top-level (HostRoot) component if ReactFeatureFlags.logTopLevelRenders is enabled.

Couple of concerns / open questions about this approach:

• Is this equivalent (enough) to the timing information stack logs?
• Are we concerned about timing in the case of bailouts caused by higher-priority updates? What about work that gets broken up across frames?

Update: This implementation potentially logs a single time entry for multiple roots, since workLoop calls findNextUnitOfWork which searches across roots. That's probably not what we want. Maybe we could only enable this logging when useSyncScheduling is enabled?

[acdlite]

@spicyj What is logTopLevelRenders intended to measure? Time to initial render?

[bvaughn]

I think the intent is to annotate the Chrome Timeline view to make it easier to match frames to React components. Not just initial render but any updates (given that a0f88d2 added logging to ReactUpdates and ReactMount).

[sebmarkbage]

This is really used as a FB specific feature to measure the overall initial render time when profiling.

@spicyj Does this look like the right thing we want to measure? Basically for sync/task/animation mode it will log the reconciliation time for an update to a root. For deferred work, it'll measure the time of a particular slice. Do we still need this feature for Web Speed?

I wonder if maybe we shouldn't do this for deferred priority work since those can't be measured as a single unit.

[sebmarkbage]

Let's get this in for parity. I'm not sure if it will always measure something we're interested in but we can revert later.