Use the correct paint methods for painting a section rect of RenderWidgets.

Previously, it was using a paint invalidation method. That is less
efficient, and also wrong if the renderer is a squashed layer.

BUG=408495

Review URL: https://codereview.chromium.org/523453002

git-svn-id: svn://svn.chromium.org/blink/trunk@181068 bbb929c8-8fbe-4397-9dbb-9b2b20218538