Convert enable_plugins to a buildflag header.

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index 01ae0352a0e2f59a8753c5bc01f25c4bd3db8cb2..c8c5198620e72f76b7f8e30d13839aedcd540eea 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -139,6 +139,7 @@
 #include "net/base/port_util.h"
 #include "net/base/registry_controlled_domains/registry_controlled_domain.h"
 #include "net/base/url_util.h"
+#include "ppapi/features/features.h"
 #include "services/service_manager/public/cpp/interface_provider.h"
 #include "services/service_manager/public/cpp/interface_registry.h"
 #include "skia/ext/event_tracer_impl.h"
@@ -2224,7 +2225,7 @@ void RenderThreadImpl::OnSystemColorsChanged(
 }
 
 void RenderThreadImpl::PurgePluginListCache(bool reload_pages) {
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
   // The call below will cause a GetPlugins call with refresh=true, but at this
   // point we already know that the browser has refreshed its list, so disable
   // refresh temporarily to prevent each renderer process causing the list to be