MacViews: Do not set delegating layer background color

Setting the background color on delegated layers can cause artifacts
(only non-tiled areas get the background color). This also appears
not to have been necessary.

Bug: 845968, 840173
Change-Id: If8eca7d3eb793952a9a606a2d27683b02d2c4b3f
Reviewed-on: https://chromium-review.googlesource.com/1071117
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561390}

content/browser/renderer_host/browser_compositor_view_mac.mm

@@ -310,7 +310,6 @@ void OnCompositingShuttingDown(ui::Compositor* compositor) override {}
 
 void BrowserCompositorMac::SetBackgroundColor(SkColor background_color) {
   background_color_ = background_color;
-  root_layer_->SetColor(background_color_);
   if (recyclable_compositor_)
     recyclable_compositor_->compositor()->SetBackgroundColor(background_color_);
 }

content/browser/renderer_host/render_widget_host_view_mac.mm

@@ -204,8 +204,6 @@ new BrowserCompositorMac(this, this, host()->is_hidden(),
 }
 
 void RenderWidgetHostViewMac::SetParentUiLayer(ui::Layer* parent_ui_layer) {
-  // TODO(ccameron): This path appears to be buggy.
-  return;
   if (parent_ui_layer)
     display_only_using_parent_ui_layer_ = true;
   if (browser_compositor_)