Replace WIDEVINE_CDM_AVAILABLE with a gyp define 'enable_widevine_cdm'.

build/config/features.gni

Index: build/config/features.gni
diff --git a/build/config/features.gni b/build/config/features.gni
index ac48896601a78e999142b392c24dd3f77cf1b201..25f898355aeffefbb368428b233aeada7dd723d5 100644
--- a/build/config/features.gni
+++ b/build/config/features.gni
@@ -39,6 +39,13 @@ declare_args() {
 
   # Enables proprietary codecs and demuxers; e.g. H264, MOV, AAC, and MP3.
   proprietary_codecs = false
+
+  # The Widevine CDM is enabled by default for Chrome builds, and is a platform
+  # feature on Android.
+  enable_widevine_cdm = is_chrome_branded || is_android
+
+  # Widevine is distributed as a component for Mac and Windows.
+  widevine_cdm_is_component = is_chrome_branded && (is_mac || is_win)
 }
 
 # Additional dependent variables -----------------------------------------------