fix: initialize transition start time with performance.now()

[subhamkumarr]

Details

• packages/react/src/ReactStartTransition.js: Added a now() helper (using performance.now() with a Date.now() fallback) and used it to set currentTransition.startTime.
• packages/react-reconciler/src/ReactFiberWorkLoop.js: Removed the check for transition.startTime === -1 and the subsequent assignment, as it is no longer needed.

Motivation

Previously, startTime was initialized to -1 and lazily updated upon scheduling an update. This behavior was inconsistent with useTransition and could lead to inaccurate timestamps if there was a delay between the transition start and the actual update scheduling. This change standardizes the behavior and ensures accurate timing for transition tracing.

How to Test

1. Enable enableTransitionTracing feature flag.
2. Run yarn test ReactTransitionTracing.
3. Verify that tests pass and that transition start times are reasonable (non-negative).

Checklist

• Tests passed locally (yarn test ReactTransitionTracing).
• I have addressed the TODOs in the codebase.

Fixes #35725

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[subhamkumarr]

@hoxyq @unstubbable @jackpope
could you please review this and merge it when you get a chance? Thanks!
Fixes #35725