login: Switch stylus UI features to a flag instead of hard-coded board names.

TEST=Verify new images contain stylus UI features
BUG=chromium:666939

Change-Id: Icb4ac7901ca3d61f79f70b414c926dca0a60a358
Reviewed-on: https://chromium-review.googlesource.com/412926
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
(cherry picked from commit 232c5333a23c51723557abb234ad1a658d0d80af)
Reviewed-on: https://chromium-review.googlesource.com/416228
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Jacob Dufault <jdufault@chromium.org>
Trybot-Ready: Jacob Dufault <jdufault@chromium.org>
diff --git a/login_manager/chrome_setup.cc b/login_manager/chrome_setup.cc
index e20b5d7..eac2e3b 100644
--- a/login_manager/chrome_setup.cc
+++ b/login_manager/chrome_setup.cc
@@ -215,12 +215,12 @@
     base::DeleteFile(data_dir.Append("Local State"), false);
   }
 
+  if (builder->UseFlagIsSet("stylus"))
+    builder->AddArg("--ash-enable-palette");
 
   if (builder->UseFlagIsSet("kevin")) {
     // TODO(jdufault): Remove this once quick unlock launches on all boards.
     builder->AddFeatureEnableOverride("QuickUnlockPin");
-    // TODO(jdufault): Remove once a dedicated USE flag exists for this.
-    builder->AddArg("--ash-enable-palette");
 
     builder->AddArg("--num-raster-threads=1");
     builder->AddArg("--enable-hardware-overlays=single-fullscreen");