Convert enable_plugins to a buildflag header.

chrome/common/chrome_content_client.h

Index: chrome/common/chrome_content_client.h
diff --git a/chrome/common/chrome_content_client.h b/chrome/common/chrome_content_client.h
index 0c352a6ebbc6fd233f98b99c73164cef08732a61..6825f37c672f106e495b93f50cccb1789f43aa0b 100644
--- a/chrome/common/chrome_content_client.h
+++ b/chrome/common/chrome_content_client.h
@@ -15,8 +15,9 @@
 #include "build/build_config.h"
 #include "chrome/common/origin_trials/chrome_origin_trial_policy.h"
 #include "content/public/common/content_client.h"
+#include "ppapi/features/features.h"
 
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
 #include "content/public/common/pepper_plugin_info.h"
 #endif
 
@@ -51,7 +52,7 @@ class ChromeContentClient : public content::ContentClient {
       content::PepperPluginInfo::PPP_ShutdownModuleFunc shutdown_module);
 #endif
 
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
   static void SetPDFEntryFunctions(
       content::PepperPluginInfo::GetInterfaceFunc get_interface,
       content::PepperPluginInfo::PPP_InitializeModuleFunc initialize_module,