Implementation of device metrics emulation in render view.

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
===================================================================
--- content/renderer/gpu/render_widget_compositor.cc	(revision 221161)
+++ content/renderer/gpu/render_widget_compositor.cc	(working copy)
@@ -299,7 +299,8 @@
                                                bool threaded)
     : threaded_(threaded),
       suppress_schedule_composite_(false),
-      widget_(widget) {
+      widget_(widget),
+      saved_device_scale_factor_(0.f) {
 }
 
 RenderWidgetCompositor::~RenderWidgetCompositor() {}
@@ -421,9 +422,22 @@
 }
 
 void RenderWidgetCompositor::setDeviceScaleFactor(float device_scale) {
-  layer_tree_host_->SetDeviceScaleFactor(device_scale);
+  if (saved_device_scale_factor_)
+    saved_device_scale_factor_ = device_scale;
+  else
+    layer_tree_host_->SetDeviceScaleFactor(device_scale);
 }
 
+void RenderWidgetCompositor::OverrideDeviceScaleFactor(float device_scale) {
+  if (device_scale) {
+    saved_device_scale_factor_ = deviceScaleFactor();
+    layer_tree_host_->SetDeviceScaleFactor(device_scale);
+  } else {
+    layer_tree_host_->SetDeviceScaleFactor(saved_device_scale_factor_);
+    saved_device_scale_factor_ = 0.f;
+  }
+}
+
 float RenderWidgetCompositor::deviceScaleFactor() const {
   return layer_tree_host_->device_scale_factor();
 }