GN: Set |enable_webvr| in the declare_args() block.

build/config/features.gni

Index: build/config/features.gni
diff --git a/build/config/features.gni b/build/config/features.gni
index c1245af0d42408d3bbc6fd4d296df5dace6b63a3..cc371470f56de75b71bce89e7654f5f90f7345a3 100644
--- a/build/config/features.gni
+++ b/build/config/features.gni
@@ -118,6 +118,10 @@ declare_args() {
 
   # Whether or not to use external popup menu.
   use_external_popup_menu = is_android || is_mac
+
+  # Enable WebVR support by default on Android
+  # Still requires command line flag to access API
+  enable_webvr = is_android
 }
 
 # Additional dependent variables -----------------------------------------------
@@ -170,10 +174,6 @@ enable_chromevox_next = false
 # Use brlapi from brltty for braille display support.
 use_brlapi = is_chromeos
 
-# Enable WebVR support by default on Android
-# Still requires command line flag to access API
-enable_webvr = is_android
-
 enable_configuration_policy = !is_ios
 
 enable_mac_keystone = is_mac && is_chrome_branded && is_official_build