[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 53d4216970906dc3b2a5921d98f5885acdfa648b..2587f8596cd62615ffddd0bba00e2d6661e79730 100644
--- a/views/widget/native_widget_aura.cc
+++ b/views/widget/native_widget_aura.cc
@@ -144,6 +144,9 @@ void NativeWidgetAura::InitNativeWidget(const Widget::InitParams& params) {
     gfx::NativeView parent = params.GetParent();
     if (parent)
       parent->AddTransientChild(window_);
+    // SetAlwaysOnTop before SetParent so that always-on-top container is used.
+    if (params.keep_on_top)
+      SetAlwaysOnTop(true);
     window_->SetParent(NULL);
   }
   // TODO(beng): do this some other way.
@@ -418,8 +421,32 @@ bool NativeWidgetAura::IsActive() const {
 }
 
 void NativeWidgetAura::SetAlwaysOnTop(bool on_top) {
-  // http://crbug.com/102582
-  NOTIMPLEMENTED();
+  switch (window_->type()) {
+    case aura::WINDOW_TYPE_NORMAL:
+    case aura::WINDOW_TYPE_POPUP:
+      SetNativeWindowProperty(aura::kAlwaysOnTopKey,
+          reinterpret_cast<void*>(on_top));
+
+      // Having a parent means it's added to some container already.
+      // Doing SetParent again here to make sure window_ is put in the right
+      // container.
+      // TODO(xiyuan): Optimize and only switch containers when necessary.
+      if (window_->parent()) {
+        window_->parent()->RemoveChild(window_);
+        window_->SetParent(NULL);
+      }
+
+      break;
+    case aura::WINDOW_TYPE_MENU:
+    case aura::WINDOW_TYPE_TOOLTIP:
+      // Menu and tooptip has their own container and ignores always-on-top.
+      break;
+    default:
+      NOTREACHED() << "Window " << window_->id()
+                   << " of type " << window_->type()
+                   << " does not support always-on-top.";
+      break;
+  }
 }
 
 void NativeWidgetAura::Maximize() {