Expose a public getter on a TracingAgent for background mode

Summary:
# Changelog: [Internal]

It is already in a tracing session state, we are going to use it in RuntimeTarget.

Differential Revision: D85440638

Author: hoxyq

packages/react-native/ReactCommon/jsinspector-modern/RuntimeAgent.h

@@ -99,7 +99,7 @@ class RuntimeAgent final {
  * Lifetime of this agent is bound to the lifetime of the Tracing session -
  * HostTargetTraceRecording and to the lifetime of the RuntimeTarget.
  */
-class RuntimeTracingAgent : tracing::TargetTracingAgent {
+class RuntimeTracingAgent : public tracing::TargetTracingAgent {
  public:
   explicit RuntimeTracingAgent(tracing::TraceRecordingState &state, RuntimeTargetController &targetController);
 

packages/react-native/ReactCommon/jsinspector-modern/tracing/TargetTracingAgent.h

@@ -27,6 +27,10 @@ class TargetTracingAgent {
     (void)state_;
   }
 
+  bool isRunningInBackgroundMode() {
+    return state_.mode == tracing::Mode::Background;
+  }
+
  protected:
   TraceRecordingState &state_;
 };