Convert enable_plugins to a buildflag header.

content/shell/app/shell_main_delegate.cc

Index: content/shell/app/shell_main_delegate.cc
diff --git a/content/shell/app/shell_main_delegate.cc b/content/shell/app/shell_main_delegate.cc
index 1a72b4be9dd669d475848944cd4caa673975092b..d37ca0a03473597504e5f7d6a3d62e5f8a85ddcf 100644
--- a/content/shell/app/shell_main_delegate.cc
+++ b/content/shell/app/shell_main_delegate.cc
@@ -36,6 +36,7 @@
 #include "media/base/media_switches.h"
 #include "media/base/mime_util.h"
 #include "net/cookies/cookie_monster.h"
+#include "ppapi/features/features.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/base/ui_base_paths.h"
 #include "ui/base/ui_base_switches.h"
@@ -156,7 +157,7 @@ bool ShellMainDelegate::BasicStartupComplete(int* exit_code) {
   if (command_line.HasSwitch(switches::kRunLayoutTest)) {
     EnableBrowserLayoutTestMode();
 
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
     if (!ppapi::RegisterBlinkTestPlugin(&command_line)) {
       *exit_code = 1;
       return true;