Convert enable_plugins to a buildflag header.

chrome/browser/metrics/chrome_metrics_service_client.h

Index: chrome/browser/metrics/chrome_metrics_service_client.h
diff --git a/chrome/browser/metrics/chrome_metrics_service_client.h b/chrome/browser/metrics/chrome_metrics_service_client.h
index e93720286ebfb0523b2c8950ae2b6164518110eb..ea09ca76f1a92b3bd95db3f5ce9d08856e9e9af3 100644
--- a/chrome/browser/metrics/chrome_metrics_service_client.h
+++ b/chrome/browser/metrics/chrome_metrics_service_client.h
@@ -23,6 +23,7 @@
 #include "components/omnibox/browser/omnibox_event_global_tracker.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
+#include "ppapi/features/features.h"
 
 class AntiVirusMetricsProvider;
 class ChromeOSMetricsProvider;
@@ -177,7 +178,7 @@ class ChromeMetricsServiceClient
   // MetricsService. Has the same lifetime as |metrics_service_|.
   metrics::ProfilerMetricsProvider* profiler_metrics_provider_;
 
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
   // The PluginMetricsProvider instance that was registered with
   // MetricsService. Has the same lifetime as |metrics_service_|.
   PluginMetricsProvider* plugin_metrics_provider_;