Clip paints of squashed layers to the bounds of the squashed layer.

RenderLayer::paint* methods assume that the caller will clip to the bounds of the dirt rect.

BUG=368410

Review URL: https://codereview.chromium.org/261753004

git-svn-id: svn://svn.chromium.org/blink/trunk@173154 bbb929c8-8fbe-4397-9dbb-9b2b20218538