Added BASE_EXPORT to EnabledStateObserver class

base/debug/trace_event_impl.h

Index: base/debug/trace_event_impl.h
diff --git a/base/debug/trace_event_impl.h b/base/debug/trace_event_impl.h
index 4cbb72144739043fe7a25f348dd6fc572ce722d5..f91554112257b0808186d77b6abb47cbb2a381b8 100644
--- a/base/debug/trace_event_impl.h
+++ b/base/debug/trace_event_impl.h
@@ -11,6 +11,7 @@
 #include <vector>
 
 #include "base/atomicops.h"
+#include "base/base_export.h"
 #include "base/callback.h"
 #include "base/containers/hash_tables.h"
 #include "base/gtest_prod_util.h"
@@ -493,7 +494,7 @@ class BASE_EXPORT TraceLog {
   // Enabled state listeners give a callback when tracing is enabled or
   // disabled. This can be used to tie into other library's tracing systems
   // on-demand.
-  class EnabledStateObserver {
+  class BASE_EXPORT EnabledStateObserver {
    public:
     // Called just after the tracing system becomes enabled, outside of the
     // |lock_|. TraceLog::IsEnabled() is true at this point.