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

content/browser/renderer_host/render_widget_host_view_aura.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/render_widget_host_view_aura.h"
 
 #include "base/auto_reset.h"
 #include "base/basictypes.h"
 #include "base/bind.h"
 #include "base/callback_helpers.h"
@@ skipping 1592 matching lines @@
 void RenderWidgetHostViewAura::OnPaint(gfx::Canvas* canvas) {
   // For non-opaque windows, we don't draw anything, since we depend on the
   // canvas coming from the compositor to already be initialized as
   // transparent.
   if (window_->layer()->fills_bounds_opaquely())
     canvas->DrawColor(SK_ColorWHITE);
 }
 
 void RenderWidgetHostViewAura::OnDeviceScaleFactorChanged(
     float device_scale_factor) {
-  if (!host_)
+  if (!host_ || !window_->GetRootWindow())
     return;
 
   UpdateScreenInfo(window_);
 
   const gfx::Display display = gfx::Screen::GetScreenFor(window_)->
       GetDisplayNearestWindow(window_);
   DCHECK_EQ(device_scale_factor, display.device_scale_factor());
   current_cursor_.SetDisplayInfo(display);
   SnapToPhysicalPixelBoundary();
 }
@@ skipping 862 matching lines @@
 
 ////////////////////////////////////////////////////////////////////////////////
 // RenderWidgetHostViewBase, public:
 
 // static
 void RenderWidgetHostViewBase::GetDefaultScreenInfo(WebScreenInfo* results) {
   GetScreenInfoForWindow(results, NULL);
 }
 
 }  // namespace content