Added BASE_EXPORT to EnabledStateObserver class

This CL adds a BASE_EXPORT to EnabledStateObserver class. This CL is a
part of the effort to get NetLog data into tracing. Since
EnabledStateObserver is not base exported, we would like it to be so
that we can use it outside base.

BUG=399701

Review URL: https://codereview.chromium.org/551333003

Cr-Commit-Position: refs/heads/master@{#293943}

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.