Aura desktop: Fix tab dragging within a single chrome window.

ui/aura/root_window.cc

Index: ui/aura/root_window.cc
diff --git a/ui/aura/root_window.cc b/ui/aura/root_window.cc
index 6c6d6bbde2f806dbba328489cb7595712220fa55..d3c99d77884af1eb6a6092ecacb41d60544ea4ae 100644
--- a/ui/aura/root_window.cc
+++ b/ui/aura/root_window.cc
@@ -159,6 +159,13 @@ RootWindow::~RootWindow() {
   layer()->GetAnimator()->RemoveObserver(this);
 }
 
+// static
+RootWindow* RootWindow::GetForAcceleratedWidget(
+    gfx::AcceleratedWidget widget) {
+  RootWindowHost* host = RootWindowHost::GetForAcceleratedWidget(widget);
+  return host ? host->GetRootWindow() : NULL;
+}
+
 void RootWindow::Init() {
   compositor()->SetScaleAndSize(GetDeviceScaleFactorFromMonitor(this),
                                 host_->GetBounds().size());