Summary:
@public
Add marker to show JavaScript download duration + flow arrows to show the origin
of the UI blocks being flushed.
Also fixed the condition on `RCTPerfStats`, UI and JS graphs were being created
at startup time, now they're just created on the first time they're shown.
Test Plan:
The markers:
{F22577660}
To check the FPS graph, enable it on the DevMenu, and it should appear initially
empty, instead of previously filled as before.
a885efe02d