Implementation of device metrics emulation in render view.

content/renderer/gpu/render_widget_compositor.h

Index: content/renderer/gpu/render_widget_compositor.h
===================================================================
--- content/renderer/gpu/render_widget_compositor.h	(revision 217537)
+++ content/renderer/gpu/render_widget_compositor.h	(working copy)
@@ -52,6 +52,9 @@
   int GetLayerTreeId() const;
   void NotifyInputThrottledUntilCommit();
 
+  // Pass zero to cancel overriding.
+  void OverrideDeviceScaleFactor(float device_scale);
+
   // WebLayerTreeView implementation.
   virtual void setSurfaceReady();
   virtual void setRootLayer(const WebKit::WebLayer& layer);
@@ -118,6 +121,9 @@
   bool threaded_;
   bool suppress_schedule_composite_;
   RenderWidget* widget_;
+  // This stores the original device scale to restore back after device scale
+  // override is canceled. When device scale if not overriden, this is zero.
+  float saved_device_scale_factor_;
   scoped_ptr<cc::LayerTreeHost> layer_tree_host_;
 };