Fixes compile and gn-check errors with enable_plugins=false.

content/ppapi_plugin/BUILD.gn

Index: content/ppapi_plugin/BUILD.gn
diff --git a/content/ppapi_plugin/BUILD.gn b/content/ppapi_plugin/BUILD.gn
index 902676a949c63e37dd1c1d18f4102efedf5dd7ab..d87e55b3dac8df16c79dcdffcac8387086f15d52 100644
--- a/content/ppapi_plugin/BUILD.gn
+++ b/content/ppapi_plugin/BUILD.gn
@@ -2,6 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//ppapi/features/features.gni")
+
+assert(enable_plugins, "PPAPI plugins must be enabled.")
+
 group("ppapi_plugin") {
   visibility = [ "//content/*" ]  # This is an internal content API.