Maintain minimal error response.

content/browser/renderer_host/render_widget_host_view_mac.mm

Index: content/browser/renderer_host/render_widget_host_view_mac.mm
diff --git a/content/browser/renderer_host/render_widget_host_view_mac.mm b/content/browser/renderer_host/render_widget_host_view_mac.mm
index 549f48a0de41929505e9911eb9d8626a3f36283e..abd2991c7a266f4c80056afcc7eae2dc0f809ef6 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -1240,10 +1240,10 @@ void RenderWidgetHostViewMac::CopyFromCompositingSurface(
     const gfx::Rect& src_subrect,
     const gfx::Size& dst_size,
     ReadbackRequestCallback& callback,
-    const SkColorType color_type) {
+    const SkColorType preferred_color_type) {
   if (delegated_frame_host_) {
     delegated_frame_host_->CopyFromCompositingSurface(
-        src_subrect, dst_size, callback, color_type);
+        src_subrect, dst_size, callback, preferred_color_type);
   }
 }
 
@@ -1709,10 +1709,6 @@ void RenderWidgetHostViewMac::PauseForPendingResizeOrRepaintsAndDraw() {
     browser_compositor_->accelerated_widget_mac()->EndPumpingFrames();
 }
 
-SkColorType RenderWidgetHostViewMac::PreferredReadbackFormat() {
-  return kN32_SkColorType;
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 // gfx::DisplayObserver, public: