Add pref to enable/disable palette tray.

BUG=616112
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2258553004
Cr-Commit-Position: refs/heads/master@{#414733}
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index aa7c7a0..01cb8bd 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -85,6 +85,11 @@
   ~PaletteDelegateImpl() override {}
 
   // PaletteDelegate:
+  std::unique_ptr<EnableListenerSubscription> AddPaletteEnableListener(
+      const EnableListener& on_state_changed) override {
+    on_state_changed.Run(false);
+    return nullptr;
+  }
   void CreateNote() override {}
   bool HasNoteApp() override { return false; }
   void SetPartialMagnifierState(bool enabled) override {}