Move ENABLE_PEPPER_CDMS to a buildflag header.

third_party/widevine/cdm/BUILD.gn

Index: third_party/widevine/cdm/BUILD.gn
diff --git a/third_party/widevine/cdm/BUILD.gn b/third_party/widevine/cdm/BUILD.gn
index a6c45e06cf636a60696f854523f1e0d72e49d51f..38495c19a3a29f7c1262d8c55c9750d6985f4677 100644
--- a/third_party/widevine/cdm/BUILD.gn
+++ b/third_party/widevine/cdm/BUILD.gn
@@ -2,6 +2,7 @@
 # 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("//build/config/chrome_build.gni")
 import("//build/config/features.gni")
 import("//chrome/process_version_rc_template.gni")
@@ -178,3 +179,10 @@ source_set("widevine_test_license_server") {
     ]
   }
 }
+
+buildflag_header("cdm_features") {
+  header = "cdm_features.h"
+  flags = [
+    "ENABLE_PEPPER_CDMS=$enable_pepper_cdms",
+  ]
+}