[MD] turn on MD by default for desktop linux

ui/base/material_design/material_design_controller.cc

Index: ui/base/material_design/material_design_controller.cc
diff --git a/ui/base/material_design/material_design_controller.cc b/ui/base/material_design/material_design_controller.cc
index 0ac15448fe3f56e17e181c6c71ccf9eb07155172..877b1a346e8d845e688be6f9b3d494df2929f050 100644
--- a/ui/base/material_design/material_design_controller.cc
+++ b/ui/base/material_design/material_design_controller.cc
@@ -44,6 +44,8 @@ MaterialDesignController::Mode MaterialDesignController::DefaultMode() {
     return Mode::MATERIAL_HYBRID;
   }
   return Mode::MATERIAL_NORMAL;
+#elif defined(OS_LINUX)
+  return Mode::MATERIAL_NORMAL;
 #else
   return Mode::NON_MATERIAL;
 #endif  // defined(OS_CHROMEOS)