Convert enable_plugins to a buildflag header.

chrome/app/chrome_main_delegate.cc

Index: chrome/app/chrome_main_delegate.cc
diff --git a/chrome/app/chrome_main_delegate.cc b/chrome/app/chrome_main_delegate.cc
index b33dec417a1fe9ab77c86e7868ff781748e5e2e8..473cede267adbf9bb90da6a451b68a43f3cc3d1b 100644
--- a/chrome/app/chrome_main_delegate.cc
+++ b/chrome/app/chrome_main_delegate.cc
@@ -51,6 +51,7 @@
 #include "content/public/common/content_paths.h"
 #include "content/public/common/content_switches.h"
 #include "extensions/common/constants.h"
+#include "ppapi/features/features.h"
 #include "printing/features/features.h"
 #include "ui/base/material_design/material_design_controller.h"
 #include "ui/base/resource/resource_bundle.h"
@@ -138,7 +139,7 @@
 #include "components/nacl/renderer/plugin/ppapi_entrypoints.h"
 #endif
 
-#if defined(ENABLE_PLUGINS) && (defined(CHROME_MULTIPLE_DLL_CHILD) || \
+#if BUILDFLAG(ENABLE_PLUGINS) && (defined(CHROME_MULTIPLE_DLL_CHILD) || \
     !defined(CHROME_MULTIPLE_DLL_BROWSER))
 #include "pdf/pdf.h"
 #endif
@@ -895,7 +896,7 @@ void ChromeMainDelegate::SandboxInitialized(const std::string& process_type) {
       nacl_plugin::PPP_InitializeModule,
       nacl_plugin::PPP_ShutdownModule);
 #endif
-#if defined(ENABLE_PLUGINS) && defined(ENABLE_PDF)
+#if BUILDFLAG(ENABLE_PLUGINS) && defined(ENABLE_PDF)
   ChromeContentClient::SetPDFEntryFunctions(
       chrome_pdf::PPP_GetInterface,
       chrome_pdf::PPP_InitializeModule,