Replace WIDEVINE_CDM_AVAILABLE with a gyp define 'enable_widevine_cdm'.

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 d151530b25320abad2a342bd22fab8e5cf702c7f..18308b10f98932ab9d38bb94fd7662bb4d14d314 100644
--- a/third_party/widevine/cdm/BUILD.gn
+++ b/third_party/widevine/cdm/BUILD.gn
@@ -2,17 +2,18 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/features.gni")
+
 widevine_arch = cpu_arch
 if (widevine_arch == "x86") {
   widevine_arch = "ia32"
 }
 
-if (is_android) {
-  # Always available on Android regardless of branding.
-  widevine_cdm_version_h_file = "android/widevine_cdm_version.h"
-  widevine_cdm_binary_files = []
-} else if (is_chrome_branded) {
-  if (is_chromeos) {
+if (enable_widevine_cdm) {
+  if (is_android) {
+    widevine_cdm_version_h_file = "android/widevine_cdm_version.h"
+    widevine_cdm_binary_files = []
+  } else if (is_chromeos) {
     widevine_cdm_version_h_file =
         "chromeos/$widevine_arch/widevine_cdm_version.h"
     widevine_cdm_binary_files = [ "chromeos/$widevine_arch/libwidevinecdm.so" ]
@@ -34,7 +35,7 @@ if (is_android) {
     widevine_cdm_binary_files = []
   }
 } else {
-  # No branding, use the default one.
+  # Not enabled, use the empty file.
   widevine_cdm_version_h_file = "widevine_cdm_version.h"
   widevine_cdm_binary_files = []
 }
@@ -71,7 +72,7 @@ if (widevine_cdm_binary_files == []) {
 }
 
 source_set("adapter") {
-  if (is_chrome_branded) {
+  if (enable_widevine_cdm) {
     deps = [
       ":version_h",
       ":binaries",
@@ -93,7 +94,7 @@ source_set("adapter") {
 # This target exists for tests to depend on that pulls in a runtime dependency
 # on the license server.
 source_set("widevine_test_license_server") {
-  if (is_chrome_branded && is_linux) {
+  if (enable_widevine_cdm && is_linux) {
     deps = [
       # TODO(GYP)
       #"//third_party/widevine/test/license_server/license_server.gyp:test_license_server"