Replace WIDEVINE_CDM_AVAILABLE with a gyp define 'enable_widevine_cdm'.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index f32797ccbdb171609eee79fa13af7c77980a9fc1..74785a95bf464b6991f094d60177ab5e4466e08e 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -44,6 +44,12 @@ config("feature_flags") {
   if (enable_notifications) {
     defines += [ "ENABLE_NOTIFICATIONS" ]
   }
+  if (enable_widevine_cdm) {
+    defines += [ "ENABLE_WIDEVINE_CDM=1" ]
+  }
+  if (widevine_cdm_is_component) {
+    defines += [ "WIDEVINE_CDM_IS_COMPONENT=1" ]
+  }
   if (enable_pepper_cdms) {
     # TODO(brettw) should probably be "=1"
     defines += [ "ENABLE_PEPPER_CDMS" ]