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 22707f03bb721077c3e1e9261eaefbff4b6743a0..0658e963255f831d0c5154bdaea1b2511e470fb9 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -1245,10 +1245,10 @@ 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);
   }
 }
 
@@ -1714,10 +1714,6 @@ void RenderWidgetHostViewMac::PauseForPendingResizeOrRepaintsAndDraw() {
     browser_compositor_->accelerated_widget_mac()->EndPumpingFrames();
 }
 
-SkColorType RenderWidgetHostViewMac::PreferredReadbackFormat() {
-  return kN32_SkColorType;
-}
-
 ////////////////////////////////////////////////////////////////////////////////
 // gfx::DisplayObserver, public: