Formalise ENABLE_TIMELINE_FRAMES experiment

[huntie]

Summary

Note

Paired with facebook/react-native#55941, should be synced into react-native afterwards.

Formalise the existing ENABLE_TIMELINE_FRAMES experiment, by removing the scattered globalThis.enableTimelineFrames and platform === 'android' checks, and ensuring the UI is correctly synced in both states.

This allows us to control enabling this feature from the backend (ReactNativeApplication.metadataEnabled → { unstable_frameRecordingEnabled }), as we experiment and add support per platform.

Also cleans up additional code forks in TimelinePanel.js.

Rollout plan (future)

• Development unstable_frameRecordingEnabled (this change): Backend-driven enabling of this experiment, when the corresponding React Native feature flag is enabled.
• Canary: At some point in the future when the CDP backend is semi-stable on both platforms, enable by default in RN, but preserve the ENABLE_TIMELINE_FRAMES experiment here — enabling per-user opt-in.
• Stable: Remove flag from frontend for general rollout.

Test plan

Experiment disabled

✅ No Screenshots toggle available

Experiment enabled

When unstable_frameRecordingEnabled is sent by the backend:

✅ Initial user notification for backend-touched experiment

✅ Upon refresh, Screenshots toggle is enabled in Performance panel

• This change maintains backwards compatibility with previous Local Storage data (if modifying settings, experiments, or other persisted client state).

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo following the contribution guide for Meta employees OR contribution guide.
• This commit is React Native-specific and cannot be upstreamed.