Add pref to enable/disable palette tray.

ash/common/system/chromeos/palette/palette_tray.cc

Index: ash/common/system/chromeos/palette/palette_tray.cc
diff --git a/ash/common/system/chromeos/palette/palette_tray.cc b/ash/common/system/chromeos/palette/palette_tray.cc
index 95db0eb52129b14519923333ddcda48946a8de90..3aa6bfe85e386d088db5bbdb22fa9d60d547fabe 100644
--- a/ash/common/system/chromeos/palette/palette_tray.cc
+++ b/ash/common/system/chromeos/palette/palette_tray.cc
@@ -4,6 +4,7 @@
 
 #include "ash/common/system/chromeos/palette/palette_tray.h"
 
+#include "ash/common/palette_delegate.h"
 #include "ash/common/shelf/shelf_constants.h"
 #include "ash/common/shelf/wm_shelf.h"
 #include "ash/common/shelf/wm_shelf_util.h"
@@ -117,7 +118,11 @@ class TitleView : public views::View, public views::ButtonListener {
 
 PaletteTray::PaletteTray(WmShelf* wm_shelf)
     : TrayBackgroundView(wm_shelf),
-      palette_tool_manager_(new PaletteToolManager(this)) {
+      palette_tool_manager_(new PaletteToolManager(this)),
+      weak_factory_(this) {
+  // PaletteTray should only be instantiated if the palette feature is enabled.
+  DCHECK(IsPaletteFeatureEnabled());
+
   PaletteTool::RegisterToolInstances(palette_tool_manager_.get());
 
   SetContentsBackground();
@@ -132,7 +137,12 @@ PaletteTray::PaletteTray(WmShelf* wm_shelf)
   WmShell::Get()->AddShellObserver(this);
   WmShell::Get()->GetSessionStateDelegate()->AddSessionStateObserver(this);
 
-  UpdateIconVisibility();
+  // OnPaletteEnabledPrefChanged will get called with the initial pref value,
+  // which will take care of showing the palette.
+  palette_enabled_ =
+      WmShell::Get()->palette_delegate()->AddPaletteEnableListener(
+          base::Bind(&PaletteTray::OnPaletteEnabledPrefChanged,
+                     weak_factory_.GetWeakPtr()));
 }
 
 PaletteTray::~PaletteTray() {
@@ -308,10 +318,14 @@ void PaletteTray::UpdateTrayIcon() {
   icon_->SetImage(CreateVectorIcon(icon, kShelfIconColor));
 }
 
-void PaletteTray::UpdateIconVisibility() {
-  if (!IsPaletteEnabled())
-    return;
+void PaletteTray::OnPaletteEnabledPrefChanged(bool enabled) {
+  if (!enabled)
+    SetVisible(false);
+  else
+    UpdateIconVisibility();
+}
 
+void PaletteTray::UpdateIconVisibility() {
   SessionStateDelegate* session_state_delegate =
       WmShell::Get()->GetSessionStateDelegate();