Add MetalayerMode to the palette.

ash/system/palette/palette_ids.cc

Index: ash/system/palette/palette_ids.cc
diff --git a/ash/system/palette/palette_ids.cc b/ash/system/palette/palette_ids.cc
index 1e2338f4a554eac9286940fde9d403ff70094dcf..63614e849bff960fde92f61e1e2f087f9797710f 100644
--- a/ash/system/palette/palette_ids.cc
+++ b/ash/system/palette/palette_ids.cc
@@ -21,6 +21,8 @@ std::string PaletteToolIdToString(PaletteToolId tool_id) {
       return "LASER_POINTER";
     case PaletteToolId::MAGNIFY:
       return "MAGNIFY";
+    case PaletteToolId::METALAYER:
+      return "METALAYER";
   }
 
   NOTREACHED();
@@ -53,6 +55,8 @@ PaletteTrayOptions PaletteToolIdToPaletteTrayOptions(PaletteToolId tool_id) {
       return PALETTE_LASER_POINTER;
     case PaletteToolId::MAGNIFY:
       return PALETTE_MAGNIFY;
+    case PaletteToolId::METALAYER:
+      return PALETTE_METALAYER;
   }
 
   NOTREACHED();