Convert enable_plugins to a buildflag header.

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 1ec908624b9a8e5636aa64b0dce367844a58b47b..414b41327a3ca1bad96ada221010af0abb56b06f 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -74,6 +74,7 @@
 #include "media/audio/audio_features.h"
 #include "media/base/media_switches.h"
 #include "media/midi/midi_switches.h"
+#include "ppapi/features/features.h"
 #include "printing/features/features.h"
 #include "ui/base/ui_base_switches.h"
 #include "ui/compositor/compositor_switches.h"
@@ -632,11 +633,11 @@ const FeatureEntry::Choice kEnableDefaultMediaSessionChoices[] = {
     {IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_DISABLED, "", ""},
     {IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_ENABLED,
      switches::kEnableDefaultMediaSession, ""},
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
     {IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_ENABLED_DUCK_FLASH,
      switches::kEnableDefaultMediaSession,
      switches::kEnableDefaultMediaSessionDuckFlash},
-#endif  // defined(ENABLE_PLUGINS)
+#endif  // BUILDFLAG(ENABLE_PLUGINS)
 };
 #endif  // !defined(OS_ANDROID)
 
@@ -882,7 +883,7 @@ const FeatureEntry kFeatureEntries[] = {
      IDS_FLAGS_MEDIA_DOCUMENT_DOWNLOAD_BUTTON_DESCRIPTION, kOsAndroid,
      FEATURE_VALUE_TYPE(features::kMediaDocumentDownloadButton)},
 #endif  // OS_ANDROID
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
     {"prefer-html-over-flash", IDS_FLAGS_PREFER_HTML_OVER_PLUGINS_NAME,
      IDS_FLAGS_PREFER_HTML_OVER_PLUGINS_DESCRIPTION, kOsDesktop,
      FEATURE_VALUE_TYPE(features::kPreferHtmlOverPlugins)},