Stop disabling compositing in SetBaseBackgroundColorBeforeMainFrame.

This test wants to verify that the WebView's output and background color
blend with existing content. Instead of asking the WebView to paint, which
is the old software path, we ask the root RenderLayer to paint (record)
itself in the way of the compositor asking for it to paint via the
GraphicsLayer.

R=boliu, enne, ojan
BUG=363772, 375864

Review URL: https://codereview.chromium.org/299753010

git-svn-id: svn://svn.chromium.org/blink/trunk@174630 bbb929c8-8fbe-4397-9dbb-9b2b20218538