Convert enable_plugins to a buildflag header.

chrome/app/BUILD.gn

Index: chrome/app/BUILD.gn
diff --git a/chrome/app/BUILD.gn b/chrome/app/BUILD.gn
index cbb7cd8b3ed138d2d7d0bf0a44df4d13c6b3f45e..5a588496dd5ed9651db745b813118273943b63ac 100644
--- a/chrome/app/BUILD.gn
+++ b/chrome/app/BUILD.gn
@@ -4,6 +4,7 @@
 
 import("//build/config/ui.gni")
 import("//chrome/common/features.gni")
+import("//ppapi/features/features.gni")
 import("//printing/features/features.gni")
 import("//services/service_manager/public/service_manifest.gni")
 import("//tools/grit/grit_rule.gni")
@@ -317,6 +318,7 @@ static_library("test_support") {
     "//components/startup_metric_utils/browser:lib",
     "//content/public/app:both",
     "//content/public/common",
+    "//ppapi/features",
     "//printing/features",
     "//ui/base",
   ]