Replace WIDEVINE_CDM_AVAILABLE with a gyp define 'enable_widevine_cdm'.

chromecast/renderer/key_systems_cast.cc

Index: chromecast/renderer/key_systems_cast.cc
diff --git a/chromecast/renderer/key_systems_cast.cc b/chromecast/renderer/key_systems_cast.cc
index 919ebbe802889ccc3e1006fa18f81aeaf12eb94c..4ff7a7afda8ead2187c0e622ace9b0afb7bc1548 100644
--- a/chromecast/renderer/key_systems_cast.cc
+++ b/chromecast/renderer/key_systems_cast.cc
@@ -27,7 +27,7 @@ void AddKeySystemWithCodecs(
 
 void AddChromecastKeySystems(
     std::vector< ::media::KeySystemInfo>* key_systems_info) {
-#if defined(WIDEVINE_CDM_AVAILABLE)
+#if defined(ENABLE_WIDEVINE_CDM)
   AddWidevineWithCodecs(cdm::WIDEVINE,
                         ::media::EME_CODEC_MP4_ALL,
                         key_systems_info);