Adds scroll position/scale emulation to DevTools.

content/renderer/devtools/render_widget_screen_metrics_emulator.cc

Index: content/renderer/devtools/render_widget_screen_metrics_emulator.cc
diff --git a/content/renderer/devtools/render_widget_screen_metrics_emulator.cc b/content/renderer/devtools/render_widget_screen_metrics_emulator.cc
index bad6fa72699b8e76b6a3c4fee6b52ceca3bc24dc..3f251fd4e30ca843aadc4f60550fb568a9febb5b 100644
--- a/content/renderer/devtools/render_widget_screen_metrics_emulator.cc
+++ b/content/renderer/devtools/render_widget_screen_metrics_emulator.cc
@@ -126,7 +126,18 @@ void RenderWidgetScreenMetricsEmulator::Apply() {
       gfx::ScaleToCeiledSize(original_resize_params_.new_size,
                              original_screen_info().deviceScaleFactor);
   modified_resize_params.new_size = applied_widget_rect_.size();
-  modified_resize_params.visible_viewport_size = applied_widget_rect_.size();
+
+  gfx::Size visible_viewport_size = applied_widget_rect_.size();
+  if (emulation_params_.visualViewportSize.width) {
+    visible_viewport_size.set_width(
+        emulation_params_.visualViewportSize.width);
+  }
+  if (emulation_params_.visualViewportSize.height) {
+    visible_viewport_size.set_height(
+        emulation_params_.visualViewportSize.height);
+  }
+  modified_resize_params.visible_viewport_size = visible_viewport_size;
+
   modified_resize_params.needs_resize_ack = false;
   delegate_->Resize(modified_resize_params);
 }