Bug 1364428 - Add a separate define for enabling stylo; r=rillian

This completes the separation between "build stylo" and "enable stylo"
that we started in the build system a few commits ago.

modules/libpref/init/all.js

@@ -5648,9 +5648,11 @@ pref("media.block-autoplay-until-in-foreground", false);
 pref("media.block-autoplay-until-in-foreground", true);
 #endif
 
-#ifdef MOZ_STYLO
 // Is the Servo-backed style system enabled?
+#ifdef MOZ_STYLO_ENABLE
 pref("layout.css.servo.enabled", true);
+#else
+pref("layout.css.servo.enabled", false);
 #endif
 
 // HSTS Priming

toolkit/moz.configure

@@ -737,6 +737,7 @@ def stylo(stylo_config, bindgen_config_paths, bindgen_enabled):
 
 set_config('MOZ_STYLO', delayed_getattr(stylo, 'build'))
 set_define('MOZ_STYLO', delayed_getattr(stylo, 'build'))
+set_define('MOZ_STYLO_ENABLE', delayed_getattr(stylo_config, 'enable'))
 
 set_config('MOZ_LIBCLANG_PATH', delayed_getattr(stylo, 'libclang_path'))
 set_config('MOZ_CLANG_PATH', delayed_getattr(stylo, 'clang_path'))