Use the pre-built scheduler (which includes a check for 'window' being defined in order to load the right scheduler implementation) rather than just directly importing a version of the scheduler that relies on window. Since the scheduling profiler's code runs partially in a web worker, it can't rely on window.
This commit changes scheduling profiler marks from a format like '--schedule-render-1' to '--schedule-render-1-Sync' (where 1 is the numeric value of the Sync lane). This will enable the profiler itself to show more meaningful labels for updates and render work.
The commit also refactors and adds additional tests for the scheduling profiler package.
It also updates the preprocessor to 'support' instant events. These are no-ops for us, but adding recognition of the event type will prevent profiles imported from e.g. Chrome Canary from throwing with an 'unrecognized event' error. (This will resolve issue #20767.)
* Move preprocessData into a web worker
* Add UI feedback for loading/import error states
* Terminate worker when done handling profile
* Add display density CSS variables
