Fix DCHECK with force-device-scale-factor switch enabled.

This CL fixes a DCHECK happening due to r284913. The CL changed the order
of window addition which caused window properties to update in a different
order. This is fixed by only running the DCHECKed code after the window
is parented beneath a root window.

BUG=396911

Review URL: https://codereview.chromium.org/417923003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285833 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/renderer_host/render_widget_host_view_aura.cc

@@ -1620,7 +1620,7 @@ void RenderWidgetHostViewAura::OnPaint(gfx::Canvas* canvas) {
 
 void RenderWidgetHostViewAura::OnDeviceScaleFactorChanged(
     float device_scale_factor) {
-  if (!host_)
+  if (!host_ || !window_->GetRootWindow())
     return;
 
   UpdateScreenInfo(window_);