Move ENABLE_PEPPER_CDMS to a buildflag header.

ppapi/features/BUILD.gn

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/buildflag_header.gni")
+import("//ppapi/features/features.gni")
+
+# This file is in a separate directory so all targets in the build can refer to
+# the buildflag header to get the necessary preprocessor defines without
+# bringing in all of Pepper.
+
+buildflag_header("features") {
+  header = "features.h"
+  flags = [ "ENABLE_PEPPER_CDMS=$enable_pepper_cdms" ]
+}