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 ecf32517ea0df812df4eb22c90fd2ea8af9389a9..fdb3419ef557109e10ae7e951bc6e347d650de6b 100644
--- a/third_party/widevine/cdm/BUILD.gn
+++ b/third_party/widevine/cdm/BUILD.gn
@@ -7,12 +7,11 @@ 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) {
+  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 = [
@@ -72,7 +71,7 @@ if (widevine_cdm_binary_files == []) {
 }
 
 source_set("adapter") {
-  if (is_chrome_branded) {
+  if (enable_widevine) {
     deps = [
       ":version_h",
       ":binaries",
@@ -93,7 +92,7 @@ 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 && is_linux) {
     deps = [
       # TODO(GYP)
       #"//third_party/widevine/test/license_server/license_server.gyp:test_license_server"