Convert enable_plugins to a buildflag header.

chrome/browser/download/chrome_download_manager_delegate.cc

Index: chrome/browser/download/chrome_download_manager_delegate.cc
diff --git a/chrome/browser/download/chrome_download_manager_delegate.cc b/chrome/browser/download/chrome_download_manager_delegate.cc
index eab5fae51d92e95ed5320dccb7eac6f4ddf3b00d..6363370fc0f8798faea125e5ae31cc228a623c1f 100644
--- a/chrome/browser/download/chrome_download_manager_delegate.cc
+++ b/chrome/browser/download/chrome_download_manager_delegate.cc
@@ -54,6 +54,7 @@
 #include "extensions/features/features.h"
 #include "net/base/filename_util.h"
 #include "net/base/mime_util.h"
+#include "ppapi/features/features.h"
 #include "ui/base/l10n/l10n_util.h"
 
 #if BUILDFLAG(ANDROID_JAVA_UI)
@@ -800,7 +801,7 @@ bool ChromeDownloadManagerDelegate::IsOpenInBrowserPreferreredForFile(
 
   // On Android, always prefer opening with an external app. On ChromeOS, there
   // are no external apps so just allow all opens to be handled by the "System."
-#if !defined(OS_ANDROID) && !defined(OS_CHROMEOS) && defined(ENABLE_PLUGINS)
+#if !defined(OS_ANDROID) && !defined(OS_CHROMEOS) && BUILDFLAG(ENABLE_PLUGINS)
   // TODO(asanka): Consider other file types and MIME types.
   // http://crbug.com/323561
   if (path.MatchesExtension(FILE_PATH_LITERAL(".pdf")) ||