Send custom CDP Event to Frontend to prepare for displaying a trace (#53079)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/53079

# Changelog: [Internal]

We need this to notify Frontend, so it updates the local state before receiving `Tracing.dataCollected` events.

Corresponding change in CDT fork - https://github.com/facebook/react-native-devtools-frontend/pull/199.

Reviewed By: sbuggay

Differential Revision: D79672598

fbshipit-source-id: b2928cb3942e34a1f4723516ecdf5062d4331591

packages/react-native/ReactCommon/jsinspector-modern/TracingAgent.cpp

@@ -44,6 +44,8 @@ TracingAgent::TracingAgent(
       sessionState_(sessionState),
       hostTargetController_(hostTargetController) {
   if (traceRecordingToEmit.has_value()) {
+    frontendChannel_(
+        cdp::jsonNotification("ReactNativeApplication.traceRequested"));
     emitTraceRecording(std::move(traceRecordingToEmit.value()));
   }
 }