Convert enable_plugins to a buildflag header.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 74e4cdc090ad9cd446f35e3276e6dbe559780555..f276555cfa08d2cfda98c5a421ca1b915629e7f6 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -8,6 +8,7 @@ import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
+import("//ppapi/features/features.gni")
 import("//testing/test.gni")
 import("//third_party/WebKit/public/public_features.gni")
 import("//v8/gni/v8.gni")
@@ -701,6 +702,7 @@ test("content_browsertests") {
     "//mojo/public/cpp/bindings",
     "//mojo/public/js",
     "//net:test_support",
+    "//ppapi/features",
     "//services/service_manager/public/cpp",
     "//storage/browser",
     "//testing/gmock",
@@ -1411,6 +1413,7 @@ test("content_unittests") {
     "//net:extras",
     "//net:test_support",
     "//ppapi/c",
+    "//ppapi/features",
     "//printing",
     "//skia",
     "//sql",