Add WebWidgetTestProxy.

content/renderer/render_widget.h

Index: content/renderer/render_widget.h
diff --git a/content/renderer/render_widget.h b/content/renderer/render_widget.h
index daee7170916a6f19d73599d623067aec40007639..c9587d67175cee8c1e44c33d6449e4af32cbefa4 100644
--- a/content/renderer/render_widget.h
+++ b/content/renderer/render_widget.h
@@ -127,6 +127,18 @@ class CONTENT_EXPORT RenderWidget
                                       CompositorDependencies* compositor_deps,
                                       blink::WebLocalFrame* frame);
 
+  // Used by content_layouttest_support to hook into the creation of
+  // RenderWidgets.
+  using CreateRenderWidgetFunction =
+      RenderWidget* (*)(CompositorDependencies*,
+                        blink::WebPopupType,
+                        const blink::WebScreenInfo&,
+                        bool,
+                        bool,
+                        bool);
+  static void InstallCreateHook(
+      CreateRenderWidgetFunction create_render_widget);
+
   // Closes a RenderWidget that was created by |CreateForFrame|.
   // TODO(avi): De-virtualize this once RenderViewImpl has-a RenderWidget.
   // https://crbug.com/545684