Emit synthetic Trace Events for non-bridgeless to fix trace representation (#49729)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49729

# Changelog: [Internal]

The only data source for non-bridgeless setup is JavaScript samples.

Chrome DevTools frontend is built around an assumption that thread should have at least single timeline or user timing event to be represented on a timeline view. They do it for filtering out unnecessary workers threads and other.

We will emit 2 synthetic Trace Events that should cover these requirements and recorded traces that contain only JavaScript samples should now be displayed correctly.

This is where trace bounds are calculated - https://github.com/facebookexperimental/rn-chrome-devtools-frontend/blob/3adf51aa915c2deb26f5d373751a15b4d0c8f259/front_end/models/trace/handlers/MetaHandler.ts#L169-L173

Reviewed By: huntie

Differential Revision: D70328681

fbshipit-source-id: 8eca0017d85de9ecbfb49074b439d5c4fee4aa56