[gin] Fire observer after added when recording is in progress.

gin/v8_platform.cc

Index: gin/v8_platform.cc
diff --git a/gin/v8_platform.cc b/gin/v8_platform.cc
index 6e5491bff11801397e353cb371b1a9762fffb970..4fe7c7ceced0d5f4d164ea14c548aca3aba0b1d6 100644
--- a/gin/v8_platform.cc
+++ b/gin/v8_platform.cc
@@ -217,12 +217,18 @@ class EnabledStateObserverImpl final
   }
 
   void AddObserver(v8::Platform::TraceStateObserver* observer) {
-    base::AutoLock lock(mutex_);
-    DCHECK(!observers_.count(observer));
-    observers_.insert(observer);
-    if (observers_.size() == 1) {
-      base::trace_event::TraceLog::GetInstance()->AddEnabledStateObserver(this);
+    {
+      base::AutoLock lock(mutex_);
+      DCHECK(!observers_.count(observer));
+      if (observers_.empty()) {
+        base::trace_event::TraceLog::GetInstance()->AddEnabledStateObserver(
+            this);
+      }
+      observers_.insert(observer);
     }
+    // Fire the observer if recording is already in progress.
+    if (base::trace_event::TraceLog::GetInstance()->IsEnabled())
+      observer->OnTraceEnabled();
   }
 
   void RemoveObserver(v8::Platform::TraceStateObserver* observer) {