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 bd803ef7884e4d0b6012d18b01e1b700e078dd6f..b82f355da3b1970939cba87e8ad28fb28ff71dd8 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -51,6 +51,12 @@ config("feature_flags") {
     # TODO(brettw) should probably be "=1"
     defines += [ "ENABLE_BROWSER_CDMS" ]
   }
+  if (enable_widevine) {
+    defines += [ "ENABLE_WIDEVINE=1" ]
+  }
+  if (widevine_cdm_is_component) {
+    defines += [ "WIDEVINE_CDM_IS_COMPONENT=1" ]
+  }
   if (enable_plugins) {
     defines += [ "ENABLE_PLUGINS=1" ]
   }