[Aura] Support always-on-top top level window.

views/widget/native_widget_aura.cc

Index: views/widget/native_widget_aura.cc
diff --git a/views/widget/native_widget_aura.cc b/views/widget/native_widget_aura.cc
index a9c88e7adb53c7a9ed4b46ac55e922f7adda0ab3..05dc01c758bd753cceb9969ba86345c08a77db75 100644
--- a/views/widget/native_widget_aura.cc
+++ b/views/widget/native_widget_aura.cc
@@ -86,6 +86,8 @@ void NativeWidgetAura::InitNativeWidget(const Widget::InitParams& params) {
   if (params.type == Widget::InitParams::TYPE_CONTROL) {
     window_->SetParent(params.GetParent());
   } else {
+    if (params.keep_on_top)
+      SetAlwaysOnTop(true);
     window_->SetParent(NULL);
     gfx::NativeView parent = params.GetParent();
     if (parent)
@@ -355,7 +357,9 @@ bool NativeWidgetAura::IsActive() const {
 }
 
 void NativeWidgetAura::SetAlwaysOnTop(bool on_top) {
-  NOTIMPLEMENTED();
+  SetNativeWindowProperty(aura::Window::kPropAlwaysOnTop,
+      reinterpret_cast<void*>(on_top));
+  // TODO(xiyuan): Switch containers if window_ is added to desktop already.
 }
 
 void NativeWidgetAura::Maximize() {