Convert enable_plugins to a buildflag header.

chrome/common/pref_names.h

Index: chrome/common/pref_names.h
diff --git a/chrome/common/pref_names.h b/chrome/common/pref_names.h
index 823d4ecca344ba8c180c3abb840714670ddf817c..8fcfbfc8fef7a9d683be5493d22ea9ced944e841 100644
--- a/chrome/common/pref_names.h
+++ b/chrome/common/pref_names.h
@@ -12,6 +12,7 @@
 #include "build/build_config.h"
 #include "chrome/common/features.h"
 #include "extensions/features/features.h"
+#include "ppapi/features/features.h"
 
 namespace prefs {
 
@@ -339,7 +340,7 @@ extern const char kPluginsDisabledPlugins[];
 extern const char kPluginsDisabledPluginsExceptions[];
 extern const char kPluginsEnabledPlugins[];
 extern const char kPluginsAlwaysOpenPdfExternally[];
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
 extern const char kPluginsShowDetails[];
 #endif
 extern const char kPluginsAllowOutdated[];
@@ -358,7 +359,7 @@ extern const char kShowUpdatePromotionInfoBar[];
 extern const char kUseCustomChromeFrame[];
 #endif
 extern const char kBackShortcutBubbleShownCount[];
-#if defined(ENABLE_PLUGINS)
+#if BUILDFLAG(ENABLE_PLUGINS)
 extern const char kContentSettingsPluginWhitelist[];
 #endif
 extern const char kPartitionDefaultZoomLevel[];