Paint the base background color on root before other bgLayers.

The background images set on the root element should not blend with
browser's base white color.

This patch makes RenderBox::paintFillLayers paint the base background
color outside of the transparency layer created to isolate blending.

It adds an extra param to RenderBoxModelObject::paintFillLayerExtended,
specifying if the caller wants to:
1)paint only the base background color,
2)skip painting the base background color or
3)include the base background color (this is the default mode and should
  not change the current behavior of this method).

BUG=389039

Review URL: https://codereview.chromium.org/355383002

git-svn-id: svn://svn.chromium.org/blink/trunk@181304 bbb929c8-8fbe-4397-9dbb-9b2b20218538